events: Simplify gesture event management

MetaGestureTracker has been separating the "did I handle an event?" and the
"should the event be filtered out?" questions, merge this and make
handle_event() reply to "should the event be only handled by me?".

If a sequence wasn't accepted yet by the gesture tracker, the event will
go through (eg. not handled exclusively by the gesture tracker) and it'll
still be processed by Clutter, triggering gesture actions, and maybe
changing the sequence into other state.

https://bugzilla.gnome.org/show_bug.cgi?id=733631