Started moving single player campaign logic to an abstract CampaignMode class to make it reusable in the eventual multiplayer campaign

Barotrauma/BarotraumaShared/Source/GameSession/GameModes/MultiplayerCampaign.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Xml.Linq;
+
+namespace Barotrauma
+{
+    class MultiplayerCampaign : CampaignMode
+    {
+        public MultiplayerCampaign(GameModePreset preset, object param) : 
+            base(preset, param)
+        {
+        }
+
+        public override void Save(XElement element)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}