QmlDesigner.PropertyEditor: some cleanup

Change-Id: I1805474d2c0f05b4662783972e1eb77e6097bf3b
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner.GradientModel: lock the model

Avoiding unwanted changes.

Change-Id: Ifafa9cc76101c8b9eb877a718db050d0eb6456f0
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>