QmlDesigner: Export locking in GradiantModel

Change-Id: I6a67d2a38c1603d603c4b426fb1416647b00b1b9
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlDesigner: Fix writing back of unchanged values in GradiantModel

Task-number: QTCREATORBUG-11848
Change-Id: Ia4ad5960dde8573b5b97dd7aabb97c1b418d2a58
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>