QmlDesigner.StatesEditor: create widget on demand

Otherwise the widget is created but never inserted in the widget
hierarchy, if the designer was never activated.
In this case the widget was not deleted.

Task-number: QTCREATORBUG-10169

Change-Id: Ic86d540a7044c07e272357edafd4d2f81fd1b7a4
Reviewed-by: Marco Bubke <marco.bubke@digia.com>