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/Script.cpp

@@ -284,8 +284,7 @@ void scriptclass::run()
 			}
 			if (words[0] == "musicfadeout")
 			{
-				music.fadeout();
-				music.dontquickfade = true;
+				music.fadeout(false);
 			}
 			if (words[0] == "musicfadein")
 			{