[UI] Add generics to JavaFX composites

`ICompositeFXController`s can now specify their own type (e.g.
`AnchorPane`) as well as the type of their containments. This improves
type safety and reduces the number of casts required.

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



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