Refactor aird.DiagramDescriptor to use the DiagramElement as reference instead of its UUID

It might be a little late for that discussion (although it can never be too late for improvements!), but do you think it makes sense to refactor the `aird.DiagramDescriptor` to include a reference to the actual XML element instead of just its UUID? This would allow code in `aird` that actually uses that descriptor to avoid essentially the same lookup in several places, and therefore also avoid the possibility for these lookups to be implemented slightly differently each time (see ).

Aside from that point, it might make sense for API consistency reasons to pass the `DiagramDescriptor` instead of just the `uid` part here, even though the other parts of it aren't actually used in this function.

_Originally posted by @Wuestengecko in https://github.com/DSD-DBS/py-capellambse/pull/84#discussion_r875673029_