Optimized ScriptEditor initialization when many scripts are loaded

This change avoids the editor to freeze for several seconds when a
project with lots of scripts is loaded in the editor.

It focuses on a few heavy operations previously executed on all
previously loaded scripts:
- Initialize script resource (script validation/parsing) only
on focus
- ScriptTextEditor: code editor and edit menu are added to the
scene only on focus
- Add to recent scripts only when opening new scripts
(load/save scene metadata)

editor/plugins/text_editor.cpp

@@ -164,8 +164,7 @@ String TextEditor::get_name() {
 }
 
 Ref<Texture> TextEditor::get_icon() {
-
-	return EditorNode::get_singleton()->get_object_icon(text_file.operator->(), "");
+	return EditorNode::get_singleton()->get_object_icon(text_file.ptr(), "");
 }
 
 RES TextEditor::get_edited_resource() const {
@@ -173,7 +172,8 @@ RES TextEditor::get_edited_resource() const {
 }
 
 void TextEditor::set_edited_resource(const RES &p_res) {
-	ERR_FAIL_COND(!text_file.is_null());
+	ERR_FAIL_COND(text_file.is_valid());
+	ERR_FAIL_COND(p_res.is_null());
 
 	text_file = p_res;
 
@@ -185,6 +185,16 @@ void TextEditor::set_edited_resource(const RES &p_res) {
 	code_editor->update_line_and_column();
 }
 
+void TextEditor::enable_editor() {
+	if (editor_enabled) {
+		return;
+	}
+
+	editor_enabled = true;
+
+	_load_theme_settings();
+}
+
 void TextEditor::add_callback(const String &p_function, PoolStringArray p_args) {
 }
 
@@ -282,6 +292,8 @@ void TextEditor::set_edit_state(const Variant &p_state) {
 			_change_syntax_highlighter(idx);
 		}
 	}
+
+	ensure_focus();
 }
 
 void TextEditor::trim_trailing_whitespace() {
@@ -361,15 +373,6 @@ void TextEditor::clear_edit_menu() {
 	memdelete(edit_hb);
 }
 
-void TextEditor::_notification(int p_what) {
-
-	switch (p_what) {
-		case NOTIFICATION_READY:
-			_load_theme_settings();
-			break;
-	}
-}
-
 void TextEditor::_edit_option(int p_op) {
 	TextEdit *tx = code_editor->get_text_edit();
 
@@ -624,6 +627,8 @@ void TextEditor::_make_context_menu(bool p_selection, bool p_can_fold, bool p_is
 }
 
 TextEditor::TextEditor() {
+	editor_enabled = false;
+
 	code_editor = memnew(CodeTextEditor);
 	add_child(code_editor);
 	code_editor->add_constant_override("separation", 0);