Added Media.PlayMusic to the Lua API

and make use of it on the shellmaps to fix https://github.com/OpenRA/OpenRA/issues/6071.
It would be useful if this could accept a lua callback to play when the song completes.  Right now there is no way to say "play act on instinct initially, and then random songs after it has completed", and I think that is too specific of a thing to build in to the API directly.