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

js/ui/chrome.js

@@ -335,6 +335,11 @@ Chrome.prototype = {
             this._updateVisibility();
             this._queueUpdateRegions();
         }
+
+        // Figure out where the pointer is in case we lost track of
+        // it during a grab. (In particular, if a trayicon popup menu
+        // is dismissed, see if we need to close the message tray.)
+        global.sync_pointer();
     },
 
     _updateRegions: function() {