add support for more roles in the model to allow generic delegates
provides roles that will be useable by the generic delegates
allow to retrieve the album or the artist with a generic role from models
support everything except open with generic view and delegate
allow all artsist and all albums models to know each other
this is needed to allow the models to generate proxy models linked to an
artist from an album item for example
makes it possible to open and use album or all albums from an artist views
still some things are not working well like the possibility to show the
artist of an album
I still need to report in the new components all the improvements that have gone since I started this branch