Deal with unknown flags from ClutterEvent.get_state()

When we get a ClutterModifierType from Clutter, it might contain
bits not in the enumeration. See bug 59771 for a similar problem
with GdkModifierType.

Add a wrapper Shell.get_event_state() around clutter_event_get_state()
to mask these bits out and only return approved bits.

https://bugzilla.gnome.org/show_bug.cgi?id=597735

js/ui/altTab.js

@@ -148,7 +148,7 @@ AltTabPopup.prototype = {
 
     _keyPressEvent : function(actor, event) {
         let keysym = event.get_key_symbol();
-        let shift = (event.get_state() & Clutter.ModifierType.SHIFT_MASK);
+        let shift = (Shell.get_event_state(event) & Clutter.ModifierType.SHIFT_MASK);
 
         // The WASD stuff is for debugging in Xephyr, where the arrow
         // keys aren't mapped correctly