Add a new event for plugins

EditorSlaveCreateEvent sometimes is not enought for plugins to add other
features, since that for editors, it doesnt provide any way to interact with
the window.

Adding a new EditorCreateEvent, that will allow plugins to add a new button
on the window, for instance.

Change-Id: Ieadd41d80e96d7281a39de15a5971e44120828c9