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
