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/WayPoint.cs

@@ -636,6 +636,7 @@ namespace Barotrauma
                 int i = 0;
                 foreach (MapEntity e in linkedTo)
                 {
+                    if (!e.ShouldBeSaved) continue;
                     element.Add(new XAttribute("linkedto" + i, e.ID));
                     i += 1;
                 }