Moved Playlist and FilePlaylist definitions back to hooke.playlist

Moved Playlist and FilePlaylist definitions back to hooke.playlist

We shouldn't define core classes in plugins.  This will make the
plugin clearer, separating the core functionality from the silly
plugin interface to that core functionality.  Such Builtins are useful
for the Hooke command line and GUIs, but not for people using Python
to import hooke directly.