Add some accessors to simplify common tasks for GestureAction users:
• clutter_gesture_action_get_motion_delta() to get the delta
on the X and Y axis in stage coordinates since the last motion
event, and the scalar distance travelled;
• clutter_gesture_action_get_velocity() to get an estimate of the
speed of the last motion event along the X and Y axis and as a
scalar value in pixels per millisecond.
https://bugzilla.gnome.org/show_bug.cgi?id=681648
436ebb2716
