Move INotifySold to Common and require explicit implementation

OpenRA.Mods.Common/Traits/Render/WithProductionDoorOverlay.cs

@@ -80,7 +80,7 @@ namespace OpenRA.Mods.Common.Traits.Render
 			desiredFrame = door.CurrentSequence.Length - 1;
 		}
 
-		public void Selling(Actor self) { buildComplete = false; }
-		public void Sold(Actor self) { }
+		void INotifySold.Selling(Actor self) { buildComplete = false; }
+		void INotifySold.Sold(Actor self) { }
 	}
 }