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/appDisplay.js

@@ -554,7 +554,7 @@ RunningWellItem.prototype = {
     },
 
     _onActivate: function (actor, event) {
-        let modifiers = event.get_state();
+        let modifiers = Shell.get_event_state(event);
 
         if (modifiers & Clutter.ModifierType.CONTROL_MASK) {
             this.appInfo.launch();