New gestures (part 1): Introduce implicit grabbing in Clutter

It's a beautiful Sunday for sitting outside and coding and we haven't introduced a new grabbing mechanism for at least 2 months, so why not do that?

This introduces the necessary infrastructure to implement ClutterActions for tracking pointer/touch gestures. The goal of this is to see all events of a button press->motion->release or a touch begin->update->end sequence in the ClutterAction, without having to make assumptions about a point getting invalid or disappearing beneath our feet.