Add conditional spawning component (#1069)

* Add conditional spawning component

* Remove null checks

* Remove leftover return

* Properly spawn items when game rule gets added

* Fix duplicate uids in saltern

* GameRules returns IEnumerable using yield.

Content.IntegrationTests/DummyGameTicker.cs

@@ -19,6 +19,12 @@ namespace Content.IntegrationTests
             remove { }
         }
 
+        public event Action<GameRuleAddedEventArgs> OnRuleAdded
+        {
+            add{ }
+            remove { }
+        }
+
         public void Initialize()
         {
         }
@@ -64,6 +70,11 @@ namespace Content.IntegrationTests
             return new T();
         }
 
+        public bool HasGameRule(Type type)
+        {
+            return false;
+        }
+
         public void RemoveGameRule(GameRule rule)
         {
         }