Completed the support for plugins! It is not possible to add plugins.

Not all APIs are provided yet, please request whathever you are missing.
Some example plugins are provided in demos/plugins. Just copy them to a folder in your project named addons/ and then enable them from the project settings.
Have fun!

tools/editor/editor_node.cpp

@@ -2952,16 +2952,109 @@ void EditorNode::remove_editor_plugin(EditorPlugin *p_editor) {
 
 void EditorNode::add_editor_import_plugin(const Ref<EditorImportPlugin>& p_editor_import) {
 
+	ERR_FAIL_COND( p_editor_import.is_null() );
 	editor_import_export->add_import_plugin(p_editor_import);
 	_rebuild_import_menu();
 }
 
 void EditorNode::remove_editor_import_plugin(const Ref<EditorImportPlugin>& p_editor_import) {
+	ERR_FAIL_COND( p_editor_import.is_null() );
 
 	editor_import_export->remove_import_plugin(p_editor_import);
 	_rebuild_import_menu();
 }
 
+
+void EditorNode::_update_addon_config() {
+
+	if (_initializing_addons)
+		return;
+
+	Vector<String> enabled_addons;
+
+	for(Map<String,EditorPlugin*>::Element *E=plugin_addons.front();E;E=E->next()) {
+		enabled_addons.push_back(E->key());
+	}
+
+	if (enabled_addons.size()==0) {
+		Globals::get_singleton()->set("editor_plugins/enabled",Variant());
+		Globals::get_singleton()->set_persisting("editor_plugins/enabled",false);
+	} else {
+		Globals::get_singleton()->set("editor_plugins/enabled",enabled_addons);
+		Globals::get_singleton()->set_persisting("editor_plugins/enabled",true);
+	}
+
+	project_settings->queue_save();
+
+}
+
+void EditorNode::set_addon_plugin_enabled(const String& p_addon,bool p_enabled) {
+
+	ERR_FAIL_COND(p_enabled && plugin_addons.has(p_addon));
+	ERR_FAIL_COND(!p_enabled && !plugin_addons.has(p_addon));
+
+	if (!p_enabled) {
+
+		EditorPlugin *addon = plugin_addons[p_addon];
+		memdelete(addon); //bye
+		plugin_addons.erase(p_addon);
+		_update_addon_config();
+		return;
+	}
+
+
+	Ref<ConfigFile> cf;
+	cf.instance();
+	String addon_path = "res://addons/"+p_addon+"/plugin.cfg";
+	Error err = cf->load(addon_path);
+	if (err!=OK) {
+		show_warning("Unable to enable addon plugin at: '"+addon_path+"' parsing of config failed.");
+		return;
+	}
+
+	if (!cf->has_section_key("plugin","script")) {
+		show_warning("Unable to find script field for addon plugin at: 'res://addons/"+p_addon+"''.");
+		return;
+	}
+
+	String path = cf->get_value("plugin","script");
+	path="res://addons/"+p_addon+"/"+path;
+
+	Ref<Script> script = ResourceLoader::load(path);
+
+
+	if (script.is_null()) {
+		show_warning("Unable to load addon script from path: '"+path+"'.");
+		return;
+	}
+
+	//could check inheritance..
+	if (String(script->get_instance_base_type())!="EditorPlugin") {
+		show_warning("Unable to load addon script from path: '"+path+"' Base type is not EditorPlugin.");
+		return;
+	}
+
+	if (!script->is_tool()) {
+		show_warning("Unable to load addon script from path: '"+path+"' Script is does not support tool mode.");
+		return;
+	}
+
+	EditorPlugin *ep = memnew( EditorPlugin );
+	ep->set_script(script.get_ref_ptr());
+	plugin_addons[p_addon]=ep;
+	add_editor_plugin(ep);
+
+	_update_addon_config();
+
+
+}
+
+bool EditorNode::is_addon_plugin_enabled(const String& p_addon) const {
+
+	return plugin_addons.has(p_addon);
+}
+
+
 void EditorNode::_remove_edited_scene() {
 	int new_index = editor_data.get_edited_scene();
 	int old_index=new_index;
@@ -3955,9 +4048,9 @@ void EditorNode::register_editor_types() {
 	ObjectTypeDB::register_type<EditorScript>();
 	ObjectTypeDB::register_type<EditorSelection>();
 	ObjectTypeDB::register_type<EditorFileDialog>();
-	ObjectTypeDB::register_type<EditorImportExport>();
+	//ObjectTypeDB::register_type<EditorImportExport>();
 	ObjectTypeDB::register_type<EditorSettings>();
-	ObjectTypeDB::register_type<UndoRedo>();
+	ObjectTypeDB::register_type<EditorSpatialGizmo>();
 
 
 	//ObjectTypeDB::register_type<EditorImporter>();
@@ -4360,6 +4453,51 @@ void EditorNode::_load_docks() {
 
 }
 
+
+void EditorNode::_update_dock_slots_visibility() {
+
+	VSplitContainer*splits[DOCK_SLOT_MAX/2]={
+		left_l_vsplit,
+		left_r_vsplit,
+		right_l_vsplit,
+		right_r_vsplit,
+	};
+
+
+	HSplitContainer*h_splits[4]={
+		left_l_hsplit,
+		left_r_hsplit,
+		main_hsplit,
+		right_hsplit,
+	};
+
+	for(int i=0;i<DOCK_SLOT_MAX;i++) {
+
+		if (dock_slot[i]->get_tab_count())
+			dock_slot[i]->show();
+		else
+			dock_slot[i]->hide();
+
+	}
+
+
+	for(int i=0;i<DOCK_SLOT_MAX/2;i++) {
+		bool in_use = dock_slot[i*2+0]->get_tab_count() || dock_slot[i*2+1]->get_tab_count();
+		if (in_use)
+			splits[i]->show();
+		else
+			splits[i]->hide();
+	}
+
+	for(int i=0;i<DOCK_SLOT_MAX;i++) {
+
+		if (!dock_slot[i]->is_hidden() && dock_slot[i]->get_tab_count()) {
+			dock_slot[i]->set_current_tab(0);
+		}
+	}
+}
+
+
 void EditorNode::_load_docks_from_config(Ref<ConfigFile> p_layout, const String& p_section) {
 
 	for(int i=0;i<DOCK_SLOT_MAX;i++) {
@@ -4697,6 +4835,31 @@ void EditorNode::_bottom_panel_switch(bool p_enable,int p_idx) {
 	}
 }
 
+
+void EditorNode::add_control_to_dock(DockSlot p_slot,Control* p_control) {
+	ERR_FAIL_INDEX(p_slot,DOCK_SLOT_MAX);
+	dock_slot[p_slot]->add_child(p_control);
+	_update_dock_slots_visibility();
+
+}
+
+void EditorNode::remove_control_from_dock(Control* p_control) {
+
+	Control *dock=NULL;
+	for(int i=0;i<DOCK_SLOT_MAX;i++) {
+		if (p_control->get_parent()==dock_slot[i]) {
+			dock=dock_slot[i];
+			break;
+		}
+	}
+
+	ERR_EXPLAIN("Control was not in dock");
+	ERR_FAIL_COND(!dock);
+
+	dock->remove_child(p_control);
+	_update_dock_slots_visibility();
+}
+
 void EditorNode::_bind_methods() {
 
 
@@ -4787,6 +4950,7 @@ EditorNode::EditorNode() {
 	EditorHelp::generate_doc(); //before any editor classes are crated
 	SceneState::set_disable_placeholders(true);
 
+
 	InputDefault *id = Input::get_singleton()->cast_to<InputDefault>();
 
 	if (id) {
@@ -4802,6 +4966,7 @@ EditorNode::EditorNode() {
 	singleton=this;
 	last_checked_version=0;
 	changing_scene=false;
+	_initializing_addons=false;
 
 	FileAccess::set_backup_save(true);
 
@@ -6061,7 +6226,7 @@ EditorNode::EditorNode() {
 	}
 
 
-	EditorSettings::get_singleton()->enable_plugins();
+
 	Node::set_human_readable_collision_renaming(true);
 
 
@@ -6077,6 +6242,19 @@ EditorNode::EditorNode() {
 	editor_data.set_edited_scene(0);
 	_update_scene_tabs();
 
+	{
+
+		_initializing_addons=true;
+		Vector<String> addons = Globals::get_singleton()->get("editor_plugins/enabled");
+
+		for(int i=0;i<addons.size();i++) {
+			set_addon_plugin_enabled(addons[i],true);
+		}
+		_initializing_addons=false;
+	}
+
+
+
 	_load_docks();