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.h

@@ -8,6 +8,9 @@
 
 #define musicroom(rx, ry) ((rx) + ((ry) * 20))
 
+/* The amount of "space" for the scale of the user-set volume. */
+#define USER_VOLUME_MAX 256
+
 class musicclass
 {
 public:
@@ -52,6 +55,10 @@ public:
 	int FadeVolAmountPerFrame;
 	int musicVolume;
 
+	/* 0..USER_VOLUME_MAX */
+	int user_music_volume;
+	int user_sound_volume;
+
 	bool quick_fade;
 
 	// MMMMMM mod settings