macosx: privatise instance variables and resolve class dependencies on internal implementation details

This simplifies the methods, also using a simpler storage for
dragged items.

The current playlist model directly operates on the core playlist
datastructures without proper locking for a complete playlist table
reload/update. This resulted in various ugly hacks and workarounds.

The new playlist model encapsulates the data in own objects like
in the qt interface. This allows a much easier integration with
the table view and proper updates from the core playlist.

This way, the previous playlist objects, stored in an ugly map with
pointer strings as keys, pointing to the same pointer inside a
NSValue, is obsolete finally. :-)

This mimics the current iTunes support and behavior is controlled by the same setting.

Signed-off-by: David Fuhrmann <david.fuhrmann@googlemail.com>