[SUG][PLAYER] Option to disable music in testplay mode

[namida]

Self-explanatory. Some people have mentioned that the transition from silence to loud music is jarring and/or that they'd usually have their own music playing when designing levels (but still want music to play as normal when playing packs etc).

Technically falls outside the "no new feature suggestions" but, there are three factors that have lead to me making a special case here:
1) This is very simple to implement.
2) I can very much see the benefit of it (even if I likely wouldn't use it myself).
3) It would be possible - but much less tidy - to implement this as a feature purely in the editor (namely, by having the editor modify NL's settings INI file to set the music volume to 0 when it launches testplay mode, and back to normal afterwards). I'd rather do it the clean way.

[namida]

Implemented in commit b99d736.