Factor fade amount calculation to separate function

This makes it so to reuse this code, we don't have to copy-paste it.

Additionally, I added a check for the milliseconds being 0, to avoid a
division by zero. Logically and mathematically, if the fade amount is 0
milliseconds, then that means the fade should happen instantly -
however, dividing by zero is undefined (both in math and in C/C++), so
this check needs to be added.

desktop_version/src/Music.h

@@ -19,6 +19,7 @@ public:
 	void resume(const int fadein_ms = 0);
 	void haltdasmusik(void);
 	void silencedasmusik(void);
+	void setfadeamount(const int fade_ms);
 	void fadeMusicVolumeIn(int ms);
 	void fadeout(const bool quick_fade_ = true);
 	void fadein(void);