UI: simplify crossreferences

[ilippert]

Task: add a way to generate a new entry that crossreferences the selected entry of the entry table

Rationale: A significant improvement would be to ease the workflow of generateíng a new entry that crossreferences an existing entry.

current situation:
a) copy bibref key
b) add new entry, select new entry
c) select/generate appropriate field
d) paste in bibref key

Proposed feature
a) in (context) menu for new bibtex entry, add submenu for "new entry crossreferencing currently selected entry"

based on discussion at https://discourse.jabref.org/t/seamless-crossreferencing/100

[ilippert]

I would add this feature as part of a context menu #6099

[ilippert]

I am closing this issue, in favour of continuing the discussion at #6294