Add music and sound volume config options

This adds <musicvolume> and <soundvolume> tags to unlock.vvv and
settings.vvv, so users' volume preferences will be persistent across
game sessions. This does not add the user interface to change them from
in-game; the next commit will do that.

desktop_version/src/Music.cpp

@@ -17,6 +17,9 @@ musicclass::musicclass(void)
 	musicVolume = 0;
 	FadeVolAmountPerFrame = 0;
 
+	user_music_volume = USER_VOLUME_MAX;
+	user_sound_volume = USER_VOLUME_MAX;
+
 	currentsong = 0;
 	nicechange = -1;
 	nicefade = false;