Spotimcgui: SpotimcCallbacks: Fix hangs when play token gets lost.

Stop the current stream when the playback token is lost.

This, plus some recent commits on PyspotifyCtypesProxy closes #91.
Spotimcgui: Playback: Add blocking flag to stop().
Spotimcgui: Playback: Don't stop previous song when playing a new one.

This should provide a smooth transition between songs, especially when
crossfade is enabled.

This was due to a previous quirk, but not longer required as
PyspotifyCtypesProxy does not kill streams abruptly anymore.