Split volume control into sfx and music

[Gama11]

Like already suggested in flixel-community, I think that the way flixel handles volume is too simple and that there should be two seperate volume values, FlxG.volumeMusic and FlxG.volumeSFX (or FlxG.volumeSound) instead of FlxG.volume.

While this would be a really simple change, I'm not sure what to do about the soundTray. It would be rather complex to allow it to manipulate both volumes individually. Plus, there would be more keys needed to make that happen (and the volume control already uses a lot), at least one to switch between the two volume bars. Of course, you could just make the soundTray control both volumes at once as well.

[gamedevsam]

We shouldn't mess with the soundTray at all. There should be a global volume control that affects both Sfx and Music. Adding more options to control Sfx and Music volumes separately is a good idea, but these should be multiplied by the global volume so we don't lose any of the current functionality.