Bunch of crew AI bugfixes/improvements:

- replacing empty oxygen tanks
- fixed characters only fixing leaks in the outer hull
- prioritizing large leaks over small ones
- fixed characters doing nothing after fixing all the leaks (and sinking to the bottom if they happen to be outside)
- more accurate welder aiming
- AIObjectiveGetItem makes the characters go to the cabinet/container an item is inside, not to the actual item (which may not be actually positioned at the container)

Subsurface/Source/GUI/GUIComponent.cs

@@ -173,7 +173,7 @@ namespace Barotrauma
             keyboardDispatcher = new KeyboardDispatcher(window);
         }
 
-        public T GetChild<T>()
+        public T GetChild<T>() where T : GUIComponent
         {
             foreach (GUIComponent child in children)
             {
@@ -182,7 +182,7 @@ namespace Barotrauma
 
             return default(T);
         }
-
+        
         public GUIComponent GetChild(object obj)
         {
             foreach (GUIComponent child in children)