Add a few groups actions

[tobiasdiez]

Adds:

• "Edit group"
• "Remove group keep subgroups"
• "Remove subgroups"
• Add / remove selected entries to / from group

They work in principle, but the display of the groups tree is not updated properly in some cases. I will address this in a new PR.

• 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• Check documentation status (Issue created for outdated help page at help.jabref.org?)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?

[Siedlerchr]

On general okay: Not sure about the uncommented stuff. You should delete this
And check for coday

[tobiasdiez]

Thanks for the feedback. The uncommented code is a reminder for myself (and others) of things that still have to be implemented. For example, undo/redo of group actions is not possible right now.

Will merge this...