[Fixed Position] Add image proxy

Add selection proxy for image elements to support image URL
dialog, WTD-881.

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

@@ -27,6 +27,13 @@ define(
              */
             proxy.color = new AccessorMutator(element, 'color');
 
+            /**
+             * Get and/or set the displayed text of this element.
+             * @param {string} [text] the new text (if setting)
+             * @returns {string} the text
+             */
+            proxy.text = new AccessorMutator(element, 'text');
+
             return proxy;
         }