SCUMM: Add option to enable or disable overriding music

[Thunderforge]

Right now if you override music in a SCUMM game (such as by adding one of ​The Fat Man remastered soundtracks), it always plays if the soundtrack is available.

It would be helpful when comparing to the original soundtrack (and debugging, etc) to be able to disable this override at will.

Request

1. In the Game Options under the "Audio" tab, an option exists to disable overriding music
   • This might be a checkbox, or maybe a radio button ("Pefer overridden music over original music", "Use original music only", "Use overridden music only"). Can experiment with different UI designs; the exact mechanism isn't important.
2. The default option is automatically use overriden music, if available, and fall back to original music if not.
3. That option is available in-game (F5 → Options)
4. When the option is changed, it takes effect immediately without a restart (when the next song starts?)