Quite often there are situations where multiple gestures try to
recognize, keeping track of the same set of points (for example an edge
drag gesture on the stage and a click gesture somewhere in the
scenegraph). Usually what's wanted here is that the first gesture to
move to RECOGNIZING wins over all other active gestures and "claims" the
point for itself.

We implement this by introducing a concept called "influencing". It
works by making all gestures operating on a shared set of points aware
of each other using ClutterAction->register_sequence().
ClutterGesture uses this vfunc to keep track of all other
ClutterGestures that are potentially conflicting, and keeps a list
(priv->cancel_on_recognizing) of those. As soon as the move to
RECOGNIZING happens, all gestures inside this list get moved to
CANCELLED.

To allow fine-grained control over this behavior, two APIs are
introduced:

1) on the implementation level (should_influence() and
should_be_influenced_by()): This is a vfunc that gets called as soon as
a potential conflict is detected. It's helpful when a specific gesture
always behaves the same towards another gesture, for example to make
sure a LongPress gesture never cancels a DragGesture.

2) on the gesture user level, clutter_gesture_can_not_cancel() is
introduced: This allows control for the user of a gesture to specify
that a specific instance of a gesture won't cancel another gesture.
Calling this twice so that both gestures can't cancel each other allows
for things like simultaneous recognition of a pinch-to-zoom and rotate
gesture.

Part-of: <!2389>