Add WavSoundLength to show track length in music player

Adds WavSoundLength to WavLoader, adapts MusicInfo to use length function depending on extension.
Why not just `streamLength /= channels` and again why is this hard-coded to one (which makes this code unreachable) while it is available at https://github.com/OpenRA/OpenRA/blob/bleed/OpenRA.Game/FileFormats/WavLoader.cs#L23.