Clean up UI, work on tutorial

src/items/Item.h

@@ -31,8 +31,9 @@ namespace ZL {
 		void clear() { items.clear(); }
 		size_t getCount() const { return items.size(); }
 
-		// Called whenever an item is added; receives the item id.
+		// Called whenever an item is added/removed; receives the item id.
 		std::function<void(const std::string&)> onItemAdded;
+		std::function<void(const std::string&)> onItemRemoved;
 	};
 
 } // namespace ZL