playlist: renamed global "playlist" variable to "g_playlist"

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.

src/main.c

@@ -235,7 +235,6 @@ int main(int argc, char *argv[])
 
 	event_pipe_init();
 	event_pipe_register(PIPE_EVENT_IDLE, idle_event_emitted);
-	event_pipe_register(PIPE_EVENT_PLAYLIST, syncPlayerAndPlaylist);
 
 	path_global_init();
 	mapper_init();