Added support for WAVE sound files

This adds `.WAV` file support to the `OpenRA.FileSystem` as promised in #2174. I tested it with an unconverted Dune 2000 `BUILD1.WAV` extracted using @mvi's [Sound Effect Tool](http://d2kplus.com/soundeffecttool.php). I will add support for [SOUND.RS](http://d2kplus.com/wiki/index.php?title=SOUND.RS) later so we can get rid of those non-free `mods/d2k/bits/*.aud` binaries in our SCM and release packages completely.