[UI] Refactor JavaFX composites

* `AF3FXViewParts` no longer implement the `ICompositeFXContainer`
interface. Instead they are reduced to a minimum, referencing a
(potentially hierarchic) `ICompositeFXContainer` with all the layout and
logic.
* FXML layouts are no longer loaded in the constructor of the
`CompositeFXControllerBase`, but in its `initialize` method (executed in
`getOrLoadLayout`).

Issue-Ref: 3437
Issue-Url: https://af3-developer.fortiss.org/issues/3437



Signed-off-by: Tiziano Munaro <munaro@fortiss.org>