JavaFX: Add Single-Node View support.

* Modify the constructors such that AF3FXViewParts can be created
  that only contain a single FX node/control, and do not come along
  with a FXML layout file.
* Due to signature conflicts, the css location must be given in the
  constructor. The default case is to use "null" such that the
  OS-native look is used.
* Add a method "setChildNodeFinal" that must be used to add the FX node
  to the view. It is not possible to do this in constructors since the
  FX toolkit is not initialized at this point in time.

Issue-Ref: 3463
Issue-Url: https://af3-developer.fortiss.org/issues/3463


Signed-off-by: Alexander Diewald <diewald@fortiss.org>