Moved position of add group button to HBox in groups panel UI

[DavidOWade]

Fixes JabRef#529
Would fix #8398 if the community were to decide to take this proposal over its competing alternatives.

Description:

Moves the "add group" button previously at the bottom of the groups GUI panel into the horizontal icon menu at the top.

Proposed change:

Implementation:

• 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.

[ThiloteE]

Maybe we could have two buttons to add a new group? one with the plus at the top as proposed here and one at the bottom?

[ThiloteE]

I put the devcall label, because the last devcall + discussion in last closed pr was opting for different solution to this issue. See #8462 (comment)

[calixtus]

@DavidOWade thank you for your interest in JabRef programming. I'm very sorry you put effort in this issue, that should have normally already been marked als closed, but wasn't, since we overlooked it in koppors repo. Even though your changes look codewise good, it was not the solution the devcall in March choose. Please feel free to choose another issue to work on. Again our apologies.