Make HstSiteMenu*Impl classes extensible (with patch)

It is possible to swap out the HstSiteMenusManager implementation via Spring configuration.

Currently, if you want to extend the site menu functionality, it is necessary to start from scratch and build the menu tree yourself.

In order to make extensions to the site menu system easier and more DRY, I suggest to introduce factory methods for all the Impl classes. This way, to extend the site menus, you only need to override the factory methods.

The attached patch applies to hst-4.2.1