Merging Entries does not allow to keep groups of all duplicated entries

[MarcelPa]

Is your suggestion for improvement related to a problem? Please describe.
When using the "merge duplicates" functionality, it is not possible to keep the assigned groups of two duplicates. I am only able to choose the groups of one of the entries. If I choose "keep merged entry", the entry is not assigned to any group afterwards.

Describe the solution you'd like
I would expect the "keep merged entry" function to assign the merged entry to all groups that any duplicated object was assigned to.

Additional context
I think it could also be up for discussion to merge the groups even when explicitly selecting one of the entries (i.e. "Keep left" or "Keep right") during merge. Personally, I cannot think of any use case that would need to merge an entry but not merge the groups.

I assume the the part of the code to fix this can be found here.

[ThiloteE]

Roughly similar issue in different context: #8836

Renaming or editing groups in the entry editor does not seem to be linked with the rest of JabRef's group system. Apparently, group of an entry that have not been created explicitly in the sidebar, are ignored on the groups sidebar.

[HoussemNasri]

Refs #6190