move the root for odx id collection to `IdentifiableElement`

[andlaus]

Currently, the IdentifiableElement class -- which represents all objects that can be referenced via the ODXLINK mechanism -- does not feature a ._build_odxlinks() method, i.e., all classes that derive from IdentifiableElement need to register their ODX ids themselfs. This should be moved to IdentifiableElement and the derived classes should be changed to call ._build_odxlinks() of their parent class.