Replaced cg_musicVolume with s_musicFading new engine cvar handling, so the
game doesn't force the config to be write each time music volume is fading-in
or fading-out. This avoid to hurt a little the system performance when player
is listening Smokin'Guns in-game music as the config was written as often as
FPS times by seconds. Also fading volume is now only set 10 times by seconds max