Further separation of client-specific code

Still not done here, just gonna push a commit now so I can pull this from elsewhere.

BarotraumaServer/Source/GameSession/GameSession.cs

@@ -0,0 +1,12 @@
+using System;
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+using System.Xml.Linq;
+
+namespace Barotrauma
+{
+    partial class GameSession
+    {
+        public const Map Map = null;
+    }
+}