Reducing usage of #if CLIENT / #elif SERVER

The server will implement some classes it probably shouldn't need because certain items or game states depend on them.

BarotraumaShared/Source/Screens/Screen.cs

@@ -0,0 +1,42 @@
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+using System;
+using System.Collections.Generic;
+
+namespace Barotrauma
+{
+    partial class Screen
+    {
+        private static Screen selected;
+        
+        public static Screen Selected
+        {
+            get { return selected; }
+        }
+
+        public virtual void Deselect()
+        {
+        }
+
+        public virtual void Select()
+        {
+            if (selected != null && selected != this)
+            {
+                selected.Deselect();
+#if CLIENT
+                GUIComponent.KeyboardDispatcher.Subscriber = null;
+#endif
+            }
+            selected = this;
+        }
+
+        public virtual Camera Cam
+        {
+            get { return null; }
+        }
+        
+        public virtual void Update(double deltaTime)
+        {
+        }
+    }
+}