[450250] Enforce implementation of hook methods only where needed.

- Ensured that within AbstractVisualPart and AbstractContentPart, all methods that are not to be implemented by clients in all possible cases (e.g. addChildVisual() is only called if children are present) are no longer provided as abstract but concrete operations, throwing an UnsupportedOperationException. This way, clients will not have to overwrite a lot of methods that are actually not used (which might cause harm if all those operations are by default overwritten with an empty implementation), while on the other side, a missing implementation is not silently ignored.
- Removed default behavior for adding/removing child visuals from AbstractFXContentPart. This is to be implem (which might cause harm if all those operations are by default overwritten with an empty implementation)ented indidually by its subclasses.