Xact Improvements

This PR has a few improvements to the core Xact system as well as some reorganization.

 - Xact now reuses the SoundEffect pool.
 - Initial support for Xact variation playlists.
 - Moved Xact into its own subfolder in Audio.
 - Moved out clip events into their own source files.

This is all preliminary work for https://github.com/mono/MonoGame/issues/2661.