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/messageTray.js

@@ -1831,7 +1831,7 @@ const MessageTray = new Lang.Class({
     },
 
     _onSummaryItemClicked: function(summaryItem, button) {
-        if (summaryItem.source.handleSummaryClick()) {
+        if (summaryItem.source.handleSummaryClick(button)) {
             if (summaryItem.source.keepTrayOnSummaryClick)
                 this._setClickedSummaryItem(null);
             else