playlist overhaul day 2

[Soren Harward]

Thanks to help from Nikolaj, I've fixed the two major crasher bugs with my 
playlist overhaul: when Amarok is started with an empty playlist, and when all 
the playlist items are removed.  I've also renamed PMGroupingProxy to 
GroupingProxy and moved it into the main playlist dir, per Ian's suggestion.

If I understand correctly, the most significant objection to my commit is the 
replacement of the playlist view, and the subsequent loss of the animations.  
My commit does not remove the existing GraphicsView, and I've already pretty 
much ported the GraphicsView to the new playlist model, so if the decision is 
made to return to the GraphicsView, it won't be terribly difficult to make the 
change in the code.

Given that the primary objection is something that can fairly easily be undone, 
and considering the major improvements in other areas (such as undo/redo and 
the navigators), I'm going to go ahead and commit the code later this 
afternoon.  I know that it's not entirely bug-free, but at this point I think 
it's best to have more eyes and brains than just mine working on this code.

-- 
Soren Harward
stharward at gmail.com