Refactor FolderShortcutPlugin

The foldershortcutplugin uses a (web-page) dialog that stores direct references to the plugin. (it is an non-static inner class) This makes the dialog page pull in the main page as well when it is serialized. Furthermore, the implementation is now highly dependent on the wicket serialization strategy.

The solution is to make the dialog a static inner class and store a ServiceReference to the plugin.