Move music to clients and add option to toggle music

[oznogon]

• Move threat estimation from server to crew screens.
• This also moves music playback and switching from server to clients, which resolves at least part of Headless server playing audio #212 (headless servers playing music).
• Create music_enabled option in options.ini and the options screen. Values:
• 0: Client never plays music during game.
• 1: Client always plays music during game.
• 2 (default): Client plays music only when running the main screen.
• Add music_enabled selector to option menu.

I'm not sure if this introduces issues with Android, which doesn't bundle music.

[daid]

The implementation looks mostly sound. I just have a small problem with the default, the option menu now specifies a default of "2", but if another code path reads it first it does not have this default yet, and thus defaults to an empty string.

If there is no music, (as on android) you can still safely request a music file set, it just does nothing.

[oznogon]

Thanks, @daid! I added a commit to have PreferencesManager check for that setting in main.cpp, and set a default if the setting is empty.