XAct Volume Improvements

This PR fixes the XAct volume implementation.

 - XAct sounds are no longer affected by `SoundEffect.MasterVolume`.
 - XAct sounds can have a volume greater than 1.0.
 - XAct sounds better obey the `AudioCategory.Volume`.

To do this I had to remove support for XAudio2 using the mastering voice to set the `SoundEffect.MasterVolume`.  Instead it works like all the other platforms and updates the volume on each `SoundEffectInstance` when the `SoundEffect.MasterVolume` changes.