Bring EditorUI (controller) back

[pjasiun]

There are some reasons to have editor UI controller class:

• some methods and properties like _createEditableUIView, get editableElement(), featureComponents does not belong to the view, they could be moved to the Editor main controller, but then it would handle too much,
• engine bring its controllers, so UI could do it too, it would create a nice separation where there are separated controllers to handle engine and UI and the main Editor class only bind these pieces together,
• the editor API for plugins should be well defined, so every developer should know what classes/properties should be brought to make plugins works fine with his editor. The View should not be a part of such API, so featureComponents should be defined in a better place.

[Reinmar]

DUP of #119.