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

src/shell-global.h

@@ -103,10 +103,11 @@ MetaRectangle *shell_global_get_primary_monitor (ShellGlobal  *global);
 int            shell_global_get_primary_monitor_index (ShellGlobal  *global);
 MetaRectangle *shell_global_get_focus_monitor   (ShellGlobal  *global);
 
-void shell_global_get_pointer (ShellGlobal         *global,
-                               int                 *x,
-                               int                 *y,
-                               ClutterModifierType *mods);
+void shell_global_get_pointer  (ShellGlobal         *global,
+                                int                 *x,
+                                int                 *y,
+                                ClutterModifierType *mods);
+void shell_global_sync_pointer (ShellGlobal         *global);
 
 GSettings *shell_global_get_settings (ShellGlobal *global);