[Fixed Position] Implement remove

Implement remove button for selected elements in fixed
position view. WTD-879.

platform/features/layout/src/FixedController.js

@@ -93,8 +93,28 @@ define(
 
             // Decorate elements in the current configuration
             function refreshElements() {
-                elementProxies = (($scope.configuration || {}).elements || [])
-                        .map(makeProxyElement);
+                // Cache selection; we are instantiating new proxies
+                // so we may want to restore this.
+                var selected = selection && selection.get(),
+                    elements = (($scope.configuration || {}).elements || []),
+                    index = -1; // Start with a 'not-found' value
+
+                // Find the selection in the new array
+                if (selected !== undefined) {
+                    index = elements.indexOf(selected.element);
+                }
+
+                // Create the new proxies...
+                elementProxies = elements.map(makeProxyElement);
+
+                // Clear old selection, and restore if appropriate
+                if (selection) {
+                    selection.deselect();
+                    if (index > -1) {
+                        selection.select(elementProxies[index]);
+                    }
+                }
+
                 // TODO: Ensure elements for all domain objects?
             }
 

platform/features/layout/src/LayoutSelection.js

@@ -75,6 +75,11 @@ define(
                 return (obj === selected) || (obj === proxy);
             }
 
+            // Getter for current selection
+            function get() {
+                return selected;
+            }
+
             // Start with the proxy selected
             selection.push(proxy);
 
@@ -95,6 +100,11 @@ define(
                  * @returns {boolean} true if selection changed
                  */
                 deselect: deselect,
+                /**
+                 * Get the currently-selected object.
+                 * @returns the currently selected object
+                 */
+                get: get,
                 /**
                  * Clear the selection, including the proxy, and dispose
                  * of this selection scope. No other calls to methods on

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

@@ -17,6 +17,7 @@ define(
          */
         function ElementProxy(element, index, elements) {
             return {
+                element: element,
                 x: new Accessor(element, 'x'),
                 y: new Accessor(element, 'y'),
                 z: new Accessor(element, 'z'),