Decouple EditorInterface from EditorPlugin

- Simplify some includes in the process.
- Also exposes EditorInterface.movie_maker_enabled as a property.

editor/plugins/canvas_item_editor_plugin.h

@@ -40,6 +40,7 @@ class CanvasItemEditorViewport;
 class ConfirmationDialog;
 class EditorData;
 class EditorZoomWidget;
+class EditorSelection;
 class HScrollBar;
 class HSplitContainer;
 class MenuButton;