Refactor ModelElementTreeViewer to enable use in another scenario

* Restrict input to INamedElement, only
* Add expandItem()
* Define ModelElementUIProvider as explicit (inner) type
  * Enable subclassing if needed
  * Use model element handler to obtain name

Issue-Ref: 3709
Issue-Url: https://af3-developer.fortiss.org/issues/3709



Signed-off-by: Simon Barner <barner@fortiss.org>