gesture-tracker: Add meta_gesture_tracker_get_n_current_touches()

Due to the way the MetaGestureTracker processes every touch event, this
will tell as closely to Clutter as possible the current number of touches
happening on the stage.

Even though, this is subject to windowing behavior, on X11, rejected touches
will be soon followed by a XI_TouchEnd event, so the compositor will stop
seeing touch sequences that are still operating on clients. On wayland, touch
sequences are processed by the compositor during all their lifetime, so these
will stay on the MetaGestureTracker with META_SEQUENCE_PENDING_END state, yet
still tracked.

https://bugzilla.gnome.org/show_bug.cgi?id=733631
display: Add meta_display_is_pointer_emulating_sequence()

This function tells the obvious on X11, and implements a similar mechanism
on wayland to determine the "pointer emulating" sequence, or one to stick
with when implementing single-touch behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=733631
window: Implement single-touch window dragging

On X11 this works because only emulated pointer events are listened for. On
wayland, the single touch behavior must be enforced in touch events, ignoring
every other sequence.

https://bugzilla.gnome.org/show_bug.cgi?id=733631
wayland: Clear hashtable pointers on meta_wayland_touch_release()

Just in case they are poked while no touch interface is available;

https://bugzilla.gnome.org/show_bug.cgi?id=733631
wayland: Handle window drags for touch events

The grabbing state is now checked for both pointer/touch devices
within the seat, and the grab start coordinates returned by
meta_wayland_seat_get_grab_info().

https://bugzilla.gnome.org/show_bug.cgi?id=733631
events: Simplify gesture event management

MetaGestureTracker has been separating the "did I handle an event?" and the
"should the event be filtered out?" questions, merge this and make
handle_event() reply to "should the event be only handled by me?".

If a sequence wasn't accepted yet by the gesture tracker, the event will
go through (eg. not handled exclusively by the gesture tracker) and it'll
still be processed by Clutter, triggering gesture actions, and maybe
changing the sequence into other state.

https://bugzilla.gnome.org/show_bug.cgi?id=733631