QmlDesigner.PropertyEditor: No transactions based on focus

This is to fragile and allows many opportunities for serious bugs.

Change-Id: I2e49c37db4c1dd2c55000013d8ec7e0978d1ef89
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Marco Bubke <marco.bubke@digia.com>