Add toolbar_title and toolbar_items functions for lazy loading
fix kwargs for toolbar_item()
Allow passing title to Group
Fix using Group in the appsettings
Add self-check to Group object
Allow dotted paths in Groups too.

Tried to protect against strings, but that gave really unexplainable
errors messages, as it would state that "staff_toolbar.items.Foo"
would not be a callable which is confusing.
README: add configuration API