JFX: Do not create controller duplicates

* Load the controller of an FXML layout programatically such that the
  controller is only loader when a ViewPart is constructed. If this is
  not done, the controller is initialized a second time by evaluating
  the controller specified in the FXML file.
* Fix an NPE in the the search function for child nodes of a FXML file.

Issue-Ref: 3482
Issue-Url: https://af3-developer.fortiss.org/issues/3482


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