Implement merging groups in the three way merge UI

[HoussemNasri]

Closes #8024
Depends on #8945
Contributes to #6190

It appears that every time a field changes, an event bus marks the library as edited. The library will remain marked as edited even after I undo groups merging and cancel the merge dialog. Right now, I can't think of a way to fix this.

Edit
Keywords and Comments can be merged too.

bandicam.2022-07-09.04-40-39-815.mp4

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[calixtus]

About undo/redo and the dirty flag for the library: Don't worry about it, undo is partially broken and needs some rethinking. Part of this process is to implement every possible action as a simplecommand (CommandPattern) to make it easier to distinguish it someday between UndoableCommand and SimpleCommand.
But this is far beyond your GSoC project.