keyboard: Keep tray after clicking summary item

Currently if a summary item signals that it has handled a click
itself, the tray hides itself. This behavior is wrong for the
On-Screen-Keyboard, which appears as a unit with the tray, so add
a property to opt-out of the default behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=683546

js/ui/keyboard.js

@@ -565,6 +565,7 @@ const KeyboardSource = new Lang.Class({
     _init: function(keyboard) {
         this._keyboard = keyboard;
         this.parent(_("Keyboard"), 'input-keyboard-symbolic');
+        this.keepTrayOnSummaryClick = true;
     },
 
     handleSummaryClick: function() {