music: API change

- Music API change
- Music drops its paused status when a new song is played
- client.ini's `audio.musicVolume` preference continues to affect Music
  songs even when the volume is changed by Music API from a script
- Music songs can be garbage collected

Remove `bool audio.audioEnabled` preference from client.ini.  It no
longer makes sense to have it now that we have sound InProgress events,
which need to load a sound and determine its length.  The sound
subsystem must always be initialized.