shellEntry: Destroy entry menus when the entry actors are destroyed

This ensures that a menu and manager aren't sticking around.

https://bugzilla.gnome.org/show_bug.cgi?id=697295

js/ui/shellEntry.js

@@ -171,4 +171,10 @@ function addContextMenu(entry, params) {
     entry.connect('button-press-event', Lang.bind(null, _onButtonPressEvent, entry));
 
     entry.connect('popup-menu', Lang.bind(null, _onPopup, entry));
+
+    entry.connect('destroy', function() {
+        entry.menu.destroy();
+        entry.menu = null;
+        entry._menuManager = null;
+    });
 }