Clean up don't-quick-fade signaling

First, the variable has been inverted, because it's bad practice to have
booleans with negative names. Secondly, instead of magically setting a
signaling variable when calling fadeout(), fadeout() now has a parameter
to set the quick_fade attribute, which is much cleaner than doing the
magical assignment that could potentially look unrelated.

desktop_version/src/Music.h

@@ -18,7 +18,7 @@ public:
 	void haltdasmusik();
 	void silencedasmusik();
 	void fadeMusicVolumeIn(int ms);
-	void fadeout();
+	void fadeout(const bool quick_fade_ = true);
 	void fadein();
 	void processmusicfadein();
 	void processmusic();
@@ -43,7 +43,7 @@ public:
 	int FadeVolAmountPerFrame;
 	int musicVolume;
 
-	bool dontquickfade;
+	bool quick_fade;
 
 	// MMMMMM mod settings
 	bool mmmmmm;