Docking port fixes again. Closes #551

- Non-saveable entities aren't removed from the linkedTo-list during saving, because that causes gaps to become unlinked with the hulls between docking ports when starting a campaign round.
- Hull rechecks are disabled on docking port gaps, because moving linked subs into place may cause the gaps to become unlinked if they're positioned in a certain way (for example in some subs the hulls between the docking ports end up inside the gap).
- Dockingports only connect the waypoints linked to their gap to each other, instead of every waypoint that happens to be inside the gap.

Barotrauma/BarotraumaShared/Source/Map/Gap.cs

@@ -135,13 +135,21 @@ namespace Barotrauma
         {
             base.Move(amount);
 
-            FindHulls();
+            if (!DisableHullRechecks) FindHulls();
         }
 
         public static void UpdateHulls()
         {
             foreach (Gap g in GapList)
             {
+                for (int i = g.linkedTo.Count - 1; i >= 0; i--)
+                {
+                    if (g.linkedTo[i].Removed)
+                    {
+                        g.linkedTo.RemoveAt(i);
+                    }
+                }
+
                 if (g.DisableHullRechecks) continue;
                 g.FindHulls();
             }