Fix a few group related issues

[tobiasdiez]

This PR fixes #2334, #1873 and #1681.
Since I didn't understood the groups code, I also refactored it. Main changes:

• Split KeywordGroup class into WordKeywordGroup, which splits field content into words and then matches them, and RegexKeywordGroup that uses a regular expression for this job.
• Extract serialization of groups to GroupSerializer (previously every group decided about its own serialization via the toString method)
• Move a few things from model and logic to gui since they mostly contained ui-related stuff (e.g. GroupDesrciptions)

---

• Change in CHANGELOG.md described
• Tests created for changes
• Screenshots added (for bigger UI changes)
• Manually tested changed features in running JabRef
• Check documentation status (Issue created for outdated help page at help.jabref.org?)
• If you changed the localization: Did you run gradle localizationUpdate?

[lenhard]

The groups code is a mess :-( Thanks for being willing to put up with this!

Unfortunately, the amount of changes is too large for meaningful review and my knowledge on groups and the related code is too limited. I would trust your ability, and the amount of tests that you added here together with the working build. Cosmetic changes can be made on a later occasion. I am sure this is not the last visit to the groups code.

Since this is on your private branch, we have no build on builds.jabref.org. If we had, we could ask the people that reported the issues to try out this version. Any chance of getting a build for this?

[koppor]

Why not using PlantUML for documenting?

Which tool did you use?

[tobiasdiez]

IntelliJ's built-in one

[koppor]

Decision: merge so that many people will be able to test it. We can include code review comments later in a separate PR.

I am very aware of the risk. Refs #1495.

[koppor]

• dragging an entry to a group does not refresh the main table. (keyword-based group)
• context menu shows "Remove group, keep subgroups" even though there are no subgroups
• deletion of a static group does not remove the assignment of entries to it
• clicking on a group and pressing Del triggers the dialog "Do you want to remove the selcted entries". If I press Cursor Down, the next group is selected. Thus Del should really apply to the current group.

Nothing important or me. Think, this was also happening using the old groups code.