messageTray: Set can_focus to true for SummaryItem

https://bugzilla.gnome.org/show_bug.cgi?id=681519

js/ui/messageTray.js

@@ -1265,6 +1265,7 @@ const SummaryItem = new Lang.Class({
                                      y_fill: true,
                                      reactive: true,
                                      button_mask: St.ButtonMask.ONE | St.ButtonMask.TWO | St.ButtonMask.THREE,
+                                     can_focus: true,
                                      track_hover: true });
 
         this._sourceBox = new St.BoxLayout({ style_class: 'summary-source' });