Added map revision property.

Downloading multiple revisions make map choice confusing for host and players. This fixes the problem with an optional Revision property.
I just mean you're displaying `lobby.Map.Author` twice and there is no reason to do that.
Even if revision number is 1 (or 0, however you want to start it), imo, that should be displayed too.

So just `author.GetText = () => "Rev.{0} by {1}".F(lobby.Map.Revision, lobby.Map.Author)` is fine.