Migrate to Open.Nat version 2.0.16

Move the dependency from all libraries to game engine only.
Initialize after the renderer setup to ensure a visible window.

OpenRA.Game/Settings.cs

@@ -59,14 +59,9 @@ namespace OpenRA
 		[Desc("Set this to false to disable UPnP even if compatible devices are found.")]
 		public bool AllowPortForward = true;
 
-		public bool NatDeviceAvailable = false; // internal check if discovery succeeded
-
 		[Desc("Time in milliseconds to search for UPnP enabled NAT devices.")]
 		public int NatDiscoveryTimeout = 1000;
 
-		[Desc("Print very detailed logs for debugging issues with routers.")]
-		public bool VerboseNatDiscovery = false;
-
 		[Desc("Starts the game with a default map. Input as hash that can be obtained by the utility.")]
 		public string Map = null;