[amarok] src: avoid code duplication in user playlists; fixes to playlist browser

[Matěj Laitl]

On 11. 4. 2012 Bart Cerneels wrote:
> Big one, took a while to understand everything that was happening.
> Perhaps splitting up the code deduplication and inline rename would
> have been a better option.

You're right, sorry about that. My commit-making part of the brain was having 
a walk or something.

> At first I thought big No-No for the inline rename, but I understand
> its for consistency. It's really something UI related intruding in the
> model, which should only deal with the data.
> In the long run I would like to move those common QActions to the view
> classes.

Yes, these would suit much better and would remove the odd "model calls view" 
interaction. I chose inline because that's what pressing F2 already did.

> Useful patch though, like that someone is paying attention to those
> classes which are potentially riddled with stupid mistakes I made.

Okay. I also suspect a bug somewhere between QtGropingProxy and 
PlaylistsByProviderModel. There are unhandled corner cases of empty 
PlaylistProviders becoming non-empty and vice versa. I think that 
QtGroupingProxy needs to diferentiate between additionally-added items and 
items from the underlying model.

	Matěj