Bug #1361450 - only add the greyed out overlay once in preview mode

On previewing a collection the greyed out overlay gets added each time
you click a collection page tab - it should only be added once.

Change-Id: Iebbd4a333438d629d252366a542981fbf2cf2fd6
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>

htdocs/js/preview.js

@@ -32,7 +32,9 @@ function showPreview(size, data) {
     setElementDimensions(preview, {'w': width});
     setElementPosition(preview, {'x': xpos, 'y': vpos.y + 200});
     showElement(preview);
-    appendChildNodes(document.body, DIV({id: 'overlay'}));
+    if (!$('overlay')) {
+        appendChildNodes(document.body, DIV({id: 'overlay'}));
+    }
 }
 
 addLoadEvent(function() {