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!

demos/plugins/custom_import_plugin/material_dialog.tscn

@@ -0,0 +1,111 @@
+[gd_scene load_steps=2 format=1]
+
+[ext_resource path="res://addons/custom_import_plugin/material_dialog.gd" type="Script" id=1]
+
+[node name="MaterialImport" type="ConfirmationDialog"]
+
+margin/right = 276.0
+margin/bottom = 154.0
+focus/ignore_mouse = false
+focus/stop_mouse = true
+size_flags/horizontal = 2
+size_flags/vertical = 2
+popup/exclusive = false
+window/title = "Silly Material Import"
+dialog/hide_on_ok = true
+script/script = ExtResource( 1 )
+__meta__ = { "__editor_plugin_screen__":"Script" }
+
+[node name="src_file" type="LineEdit" parent="."]
+
+margin/left = 19.0
+margin/top = 6.0
+margin/right = 190.0
+margin/bottom = 29.0
+focus/ignore_mouse = false
+focus/stop_mouse = true
+size_flags/horizontal = 2
+size_flags/vertical = 2
+text = ""
+max_length = 0
+editable = true
+secret = false
+
+[node name="src_browse" type="Button" parent="."]
+
+margin/left = 195.0
+margin/top = 7.0
+margin/right = 249.0
+margin/bottom = 29.0
+focus/ignore_mouse = false
+focus/stop_mouse = true
+size_flags/horizontal = 2
+size_flags/vertical = 2
+toggle_mode = false
+text = "browse"
+flat = false
+
+[node name="dst_browse" type="Button" parent="."]
+
+margin/left = 195.0
+margin/top = 47.0
+margin/right = 249.0
+margin/bottom = 69.0
+focus/ignore_mouse = false
+focus/stop_mouse = true
+size_flags/horizontal = 2
+size_flags/vertical = 2
+toggle_mode = false
+text = "browse"
+flat = false
+
+[node name="dst_file" type="LineEdit" parent="."]
+
+margin/left = 19.0
+margin/top = 46.0
+margin/right = 190.0
+margin/bottom = 69.0
+focus/ignore_mouse = false
+focus/stop_mouse = true
+size_flags/horizontal = 2
+size_flags/vertical = 2
+text = ""
+max_length = 0
+editable = true
+secret = false
+
+[node name="use_red_anyway" type="CheckBox" parent="."]
+
+margin/left = 20.0
+margin/top = 84.0
+margin/right = 144.0
+margin/bottom = 106.0
+focus/ignore_mouse = false
+focus/stop_mouse = true
+size_flags/horizontal = 2
+size_flags/vertical = 2
+toggle_mode = true
+text = "Use Red Anyway"
+flat = false
+align = 0
+
+[node name="error" type="AcceptDialog" parent="."]
+
+visibility/visible = false
+margin/right = 40.0
+margin/bottom = 40.0
+focus/ignore_mouse = false
+focus/stop_mouse = true
+size_flags/horizontal = 2
+size_flags/vertical = 2
+popup/exclusive = false
+window/title = "Alert!"
+dialog/hide_on_ok = true
+
+[connection signal="confirmed" from="." to="." method="_on_MaterialImport_confirmed"]
+
+[connection signal="pressed" from="src_browse" to="." method="_on_src_browse_pressed"]
+
+[connection signal="pressed" from="dst_browse" to="." method="_on_dst_browse_pressed"]
+
+