Unify source code names

[matthias-ronge]

At the latest since the metadata editor was expanded to include physical structure trees, the API and adjacent code no longer depict the current state and should be cleaned up. This applies to both the Parent − MediaUnit − IncludedStructuralElement object hierarchy and the names.

Current API:

Suggestion:

[matthias-ronge]

I tried to start this but it produces too many changes to make in one branch. This has to be done step by step.

[Kathrin-Huber]

Isn't this done already?

[matthias-ronge]

In parts, but not all yet