Some extensions for the o.i.req.core language

[kbirken]

This PR provides some extensions for the org.iets3.req.core language. Some of these extensions are needed for variability implementation, some are generically useful. See ticket #1495 for details.

All user-facing changes are explained in CHANGELOG.md.

Closes #1495.

[arimer]

The implentation looks fine. Maybe some small quetions:

The propertyhideEmptyChildReqsSection is at the moment part of the RequirementsChunk concept.

Does it semantically make sense to keep it as a property?

To me it looks like it is only required to manipulate the editors (show/hide cells) and might not belong (semantically) to the chunk itself. If you just want to manipulate the editor we could still use userobjects. In case you want to persist that setting (per MPS user and not in the AST) we could use PropertiesComponent

However I can't judge about the semantics of that chunk, therefor I am approving.

[kbirken]

Thanks for review and approval!

The propertyhideEmptyChildReqsSection is at the moment part of the RequirementsChunk concept.

Does it semantically make sense to keep it as a property?

Good point, it's not 100% clear if it should be a property. However, as the value of the flag is per-chunk and will be linked to the chunk's lifecycle, it likely makes sense to persist it (per chunk). So in an early stage when the chunk is edited a lot, it will be switched off (default false), and in later stages (when the chunk is "frozen"), it will be switched on to hide all the clutter (one extra line for each leaf requirement). But as it is not clear yet from real-world projects how the lifecycle should be expressed in the model, I decided to just name the flag according to its impact. We might rename it later.