keybindings: Make sure events are always reported to the grab window

We have no need for normally reported events during grabs. In fact, it
might be harmful. A plugin might grab the keyboard through
meta_begin_modal_for_plugin() and then expect events to be reported to
the grab window they provide. If meanwhile this XIGrabDevice is
issued, events might start being reported normally to one other of our
windows breaking the plugin event processing.

In particular, on an empty workspace, we set input focus to our
no_focus_window. Then, if gnome-shell calls
meta_begin_modal_for_plugin() and meta_display_freeze_keyboard(), in
that order, input events will start being reported to no_focus_window.

There are two issues with this. One is that no_focus_window isn't
selecting for XI input events and thus the server discards them
completely. But even if that is fixed, events being reported to any
window other than the one gnome-shell expects - the clutter stage
window - means that events will stop reaching it.

https://bugzilla.gnome.org/show_bug.cgi?id=701219

src/core/keybindings.c

@@ -1480,7 +1480,7 @@ grab_keyboard (MetaDisplay *display,
                               timestamp,
                               None,
                               grab_mode, grab_mode,
-                              True, /* owner_events */
+                              False, /* owner_events */
                               &mask);
 
   if (grab_status != Success)