Add WavSoundLength to show track length in music player

Adds WavSoundLength to WavLoader, adapts MusicInfo to use length function depending on extension.
AudLoader seems to be a not so good template. I have no idea why this is parsed again and separately in a static method. Have a look at https://github.com/OpenRA/OpenRA/blob/bleed/OpenRA.Game/Sound/Sound.cs#L51