core: Add MetaGestureTracker

This object tracks both touch sequences happening on the stage and
gestures attached to the stage actor. When a gesture emits
::gesture-begin, All triggering sequences and future ones will be
marked as "accepted" by the compositor, and events will be listened
for meanwhile there are active gestures.

If a sequence goes unclaimed for a short time, it will be
automatically "denied", and punted to the client or shell element
below.
display: Add a global MetaGestureTracker
events: Hook MetaGestureTracker to display event processing

Events aren't actually consumed by the MetaGestureTracker, but it
rather defines whether the event will reach clients, or the stage.
display: Set an X11 passive touch grab on the root window

Touch events will be caught first by the compositor this way,
whenever the MetaGestureTracker notifies of the accepted/rejected
state of a sequence, XIAllowTouchEvents() will be called on it
accordingly, so it is handled exclusively by the compositor or
punted to clients.
backend: Use the most recent event time on replayed touch events

When a touch sequence is passively grabbed and later rejected, events
will be replayed on the next client in propagation order, although those
events (either transformed to pointer events or not) will contain the
original timestamps, this will make grabs fail with InvalidTime if triggered
from the replayed ButtonPress/TouchBegin handler.

In order to work around this, store the most recent event time (presumably
gotten from the XI_TouchEnd caused by the passive grab being rejected), and
use that time on the events being replayed afterwards and grabs, so we don't
possibly fail with InvalidTime if those events result in a compositor grab.
frames: Keep information about the ongoing grab operation, and retry if needed.

When a passive touch grab is rejected over the frame, management is punted to
the frame itself, and pointer events emulated, but the attempt to transfer the
grab from the GDK connection to the Clutter one fails with AlreadyGrabbed, and
will fail until the Clutter connection receives the XI_TouchEnd resulting from
XIRejectTouch, gotten after the XI_ButtonPress on the GDK connection.

In order to bypass this shortcoming, store the current grab operation on the
frame as long as the button is pressed, so it is retried once on the next
motion event happening during frame dragging, that will have a recent enough
timestamp to succeed. If no grabbing succeeded, the current grab operation
data will be reset on GDK_BUTTON_RELEASE.
gesture-tracker: Implement threshold-based sequence rejection

If a sequence moves past a certain distance without being used by a
gesture, reject it so clients may see and react to it ASAP. This makes
gestures to be began by initially quasi-static touchpoints, in addition to
quasi-simultaneous.