before, the mod was removed, but the button was still active
Resolves #2018. Closes #2079.
Removes mod application when exiting back to main menu. Alternative to #1968. Closes #1961.
Closes #1757
Fixes #1208
Carousel was not aware of the disabled beatmap change state. Also it was being set too late (in an async load) so wasn't useful. It's now pre-emptively set in PlaySongSelect before loading Player.
Note that tracks' looping is never restored now. There's unfortunately no easy way around this, but it shouldn't have any side-effects for now.
Because PlaySongSelect is suspending when selecting a song, we don't actually hide ModSelectOverlay.