[Representation] Add DnD service

Add service to communicate data after/during a drag-and-drop, to allow
drag contents to be checked during dragover, and to allow full JavaScript
objects to be passed during drags (within the same window.) WTD-988.

platform/representation/src/services/DndService.js

@@ -0,0 +1,50 @@
+/*global define*/
+
+define(
+    [],
+    function () {
+        "use strict";
+
+        /**
+         * Drag-and-drop service.
+         * Supplements HTML5 drag-and-drop support by:
+         * * Storing arbitrary JavaScript objects (not just strings.)
+         * * Allowing inspection of dragged objects during `dragover` events,
+         *   etc. (which cannot be done in Chrome for security reasons)
+         * @constructor
+         * @param $log Angular's $log service
+         */
+        function DndService($log) {
+            var data = {};
+
+            return {
+                /**
+                 * Set drag data associated with a given type.
+                 * @param {string} key the type's identiifer
+                 * @param {*} value the data being dragged
+                 */
+                setData: function (key, value) {
+                    $log.debug("Setting drag data for " + key);
+                    data[key] = value;
+                },
+                /**
+                 * Get drag data associated with a given type.
+                 * @returns {*} the data being dragged
+                 */
+                getData: function (key) {
+                    return data[key];
+                },
+                /**
+                 * Remove data associated with active drags.
+                 * @param {string} key the type to remove
+                 */
+                removeData: function (key) {
+                    $log.debug("Clearing drag data for " + key);
+                    delete data[key];
+                }
+            };
+        }
+
+        return DndService;
+    }
+);