eventcontroller: Add vfuncs to (un)set widget

eventcontroller: Add vfuncs to (un)set widget

This is the first step towards refactoring how widgets deal with event
controllers.

In the future, the widget will treat controllers the same way it treats
child widgets:
1. The controllers will be created without a widget.
2. There will be gtk_widget_add/remove_controller() functions to add
   or remove controllers.
3. The widget will hold a reference to all its controllers.

This way we will ultimately be able to automate controllers with ui
files.