QmlDesigner.instances: allow fallback to item

If we know from the (text editor) meta system that an object is an
(visible) item, we always create an item as fallback.

We might not be able to instaciate the original item.

Change-Id: I6851cd7a8de68d9d09b789927c204f0617017de7
Reviewed-on: http://codereview.qt.nokia.com/829


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Marco Bubke <marco.bubke@nokia.com>