QmlDesigner.instances: create working stand-in object

Create an Item as stand-in.

Change-Id: Ic6521bcade216bbb04679eabd909d2eec3d8a366
Reviewed-on: http://codereview.qt-project.org/6076


Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>