[Fixed Position] Add fill properties

Add 'fill' properties, to allow specifying background
color in Fixed Position view. WTD-881.

platform/features/layout/src/elements/BoxProxy.js

@@ -0,0 +1,40 @@
+/*global define*/
+
+define(
+    ['./ElementProxy', './AccessorMutator'],
+    function (ElementProxy, AccessorMutator) {
+        'use strict';
+
+        /**
+         * Selection proxy for Box elements in a fixed position view.
+         * Also serves as a superclass for Text elements, since those
+         * elements have a superset of Box properties.
+         *
+         * Note that arguments here are meant to match those expected
+         * by `Array.prototype.map`
+         *
+         * @constructor
+         * @param element the fixed position element, as stored in its
+         *        configuration
+         * @param index the element's index within its array
+         * @param {Array} elements the full array of elements
+         */
+        function BoxProxy(element, index, elements) {
+            var proxy = new ElementProxy(element, index, elements);
+
+            /**
+             * Get/set this element's fill color. (Omitting the
+             * argument makes this act as a getter.)
+             * @method
+             * @memberof BoxProxy
+             * @param {string} fill the new fill color
+             * @returns {string} the fill color
+             */
+            proxy.fill = new AccessorMutator(element, 'fill');
+
+            return proxy;
+        }
+
+        return BoxProxy;
+    }
+);