XInput 2.3 adds support for "barrier events", which let us know when
a pointer barrier has been hit, and when the pointer has stopped
hitting the barrier, and lets us "release" the barrier, temporarily
letting the pointer pass through the barrier. These features can be
combined to allow for certain pointer gestures, such as "pushing"
against the bottom of the screen, or stopping the pointer on monitor
edges while dragging slowly for increased edge precision.
This commit should allow graceful fallback if servers with
XInput 2.3 aren't supported.
https://bugzilla.gnome.org/show_bug.cgi?id=677215
Currently, we have a few function wrappers in the shell for pointer
barriers. If we want to implement interactive features on barriers,
we need some sort of signal to be notified of the interactivity.
In that case, we need to make a more sophisticated object-based wrapper
for a pointer barrier. Add one, and stick it in mutter.
https://bugzilla.gnome.org/show_bug.cgi?id=677215
