Playlist browser: drag & drop and plugged playlists cleanups

This change fixes a couple of bugs:
* it is now possible to drop tracks & playlists to playlist providers
in Saved Playlists
* When a new empty playlist provider appears or disappears when Amarok is
running, the view is correctly updated.

FEATURES:
* "Crop playlist" functionality implemented using drag & drop.

CHANGES:
* Drag & drop of tracks and playlists to Saved Playlists works in all cases.

BUGFIXES:
* iPod playlists now appear even when plugged in while Amarok is running.





FIXED-IN: 2.6