MessageTray: pass keyboard events to tray icons

Synthetize XKeyEvents for clicks emulated by StButton using Return or
Space.

https://bugzilla.gnome.org/show_bug.cgi?id=687425

js/ui/keyboard.js

@@ -637,11 +637,7 @@ const KeyboardSource = new Lang.Class({
         this.keepTrayOnSummaryClick = true;
     },
 
-    handleSummaryClick: function() {
-        let event = Clutter.get_current_event();
-        if (event.type() != Clutter.EventType.BUTTON_RELEASE)
-            return false;
-
+    handleSummaryClick: function(button) {
         this.open();
         return true;
     },