macosx: deinit VLCMain, fix retain cycles for InputManager and playlist

By deinitializing VLCMain, all connected objects are supposed to
be deallocated automatically. If [VLCMain sharedInstance] is gone,
the intf is deinitalized (this was previously checked with a
nil value of VLCIntf).

Fixes some retain cycles in input manager and playlist code which
prevented these objects to be deinitalized.