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.
This will be used right before handle_event() in order to filter
out events, useful to make the previous "no touchpad events" behavior
the default, and have gesture subclasses include manually the touchpad
events they handle.
