shell-global: try to resync the pointer state after grabs

shell-global: try to resync the pointer state after grabs

If the pointer moves on or off the stage while another process has a
grab, we will lose track of it. One example of this is that if you use
a popup menu from a message tray trayicon, the tray will stay up after
the menu goes away, because the shell never saw the pointer leave it.

Add a new method shell_global_sync_pointer() that causes clutter to
recheck what actor is under the pointer and generate leave/enter
events if appropriate.

Of course, we can't actually tell for sure when another process has a
grab, so we need a heuristic of when to call this. Currently we call
it from Chrome._windowsRestacked(), which is not really the right
thing at all, but does fix the menu-from-trayicon case...

https://bugzilla.gnome.org/show_bug.cgi?id=630842