Implement cache invalidation for TreeView portlet cache.

The contents of the TreeView portlet get cached in [ftw.treeview](https://github.com/4teamwork/ftw.treeview/blob/c8b8ef9f2b77baa4ce526445a2f5b5a036de40af/ftw/treeview/view.py#L45).

Upon changes to any of the RepositoryFolders we need to invalidate this cache.