Droppable: Fix dependencies in photo manager demo

Author: scottgonzalez

demos/droppable/photo-manager.html

@@ -12,6 +12,7 @@
 	<script src="../../ui/draggable.js"></script>
 	<script src="../../ui/droppable.js"></script>
 	<script src="../../ui/resizable.js"></script>
+	<script src="../../ui/button.js"></script>
 	<script src="../../ui/dialog.js"></script>
 	<link rel="stylesheet" href="../demos.css">
 	<style>