QmlDesigner.formEditor: usablity fix

Displaying the bounding Rectangle  makes no sense for small/thin items.

Change-Id: Ia30692e0cfa2641cfe3b4962a2f54f9eafe50201
Reviewed-on: http://codereview.qt-project.org/4984


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