After the state file has been loaded, the playlist version is still
"1", and "plchanges 1" returns the whole playlist.  Fix this by
increasing the playlist version after the state file has been loaded.

Export the "g_playlist" variable, and pass it to all playlist
functions.  This way, we can split playlist.c easier into separate
parts.  The code which initializes the singleton variable is moved to
playlist_global.c.

Moved everything related to saving or loading the playlist from/to the
state file to playlist_state.c.