JFX diagram editors: Delegation factories for extensibility

* Only use delegate factories in the FX editor base. By that all editors
  are extensible.
* Move the definition of the factories of one editor from the editor to
  its bindings.
* Create the concrete factories by reflection when a FX editor is
  constructed.

Issue-Ref: 3883
Issue-Url: https://af3-developer.fortiss.org/issues/3883


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