Introduce ClutterGesture, a new ClutterAction subclass and the successor
of ClutterGestureAction that brings the necessary tools to handle
sequences of events and abstract touch and mouse gestures from those.

The big difference compared to ClutterGestureAction is that ClutterGesture
provides the implementation with point_added/moved/ended and
sequences_cancelled events and expects the implementation to move the
ClutterGesture through the ClutterGestureState state machine. This state
machine is then used internally by ClutterGesture to coordinate with other
gestures.

With the next commits, ClutterGesture will handle relationships between
conflicting gestures completely by itself, allowing the implementation or
the user to specify the details of the relationship between two gestures.

Part-of: <!2389>