gesture-action: Let subclasses override the GestureTriggerEdge handling

Let gesture subclasses override how the drag threshold should
be handled:

• CLUTTER_GESTURE_TRIGGER_NONE tells GestureAction that the gesture
  must begin immediately and there's no drag limit that will cause
  its cancellation;
• CLUTTER_GESTURE_TRIGGER_AFTER is the default GestureAction behaviour,
  where it needs to wait until the drag threshold has been exceeded
  before considering the gesture valid;
• CLUTTER_GESTURE_TRIGGER_BEFORE will make GestureAction cancel
  the gesture once the drag exceed the configured threshold.

For example, ZoomAction and RotateAction could set
CLUTTER_GESTURE_TRIGGER_NONE since the use of two fingers makes the
begin of the action more self-evident, while an hypothetical Tap
gesture may use CLUTTER_GESTURE_TRIGGER_BEFORE to cancel the tap if
the pointer moves too much.

https://bugzilla.gnome.org/show_bug.cgi?id=685028