Add 'target' and 'property' properties in Behavior

When writing a custom Behavior, I'd like to get read access to its internal QQmlProperty. It could be exposed as a target QObject property and a property QString property.

My usecase is having a SequentialAnimation as the anim of the behavior, a PropertyAction using automatically the property of the behavior (this works) and a couple other PropertyAnimation that should animate the same object but a different property. Setting the PropertyAnimation's target to the Behavior's target would help.

If a PropertyAnimation has either its target or property set, it won't use its default QQmlProperty anymore.