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/main.js

@@ -201,7 +201,7 @@ function _globalKeyPressHandler(actor, event) {
                 overview.hide();
 
             return true;
-        } else if (symbol == Clutter.F2 && (event.get_state() & Clutter.ModifierType.MOD1_MASK)) {
+        } else if (symbol == Clutter.F2 && (Shell.get_event_state(event) & Clutter.ModifierType.MOD1_MASK)) {
             getRunDialog().open();
         }
     }