Added eye candy in entry editor context menus

[calixtus]

This PR adds some eye candy in the entry editor editors context menu.

Sadly, the TextField in the FXChipView is not accessible, so this one lacks eye candy.

Still WIP, I'll see if I can fix some misplaced other menu entries.

Refs #5352

• Change in CHANGELOG.md described (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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[calixtus]

Hi @tobiasdiez , I'm currently refactoring the context menu for the field editors. It's still work in progress, but I encountered some design choices of yours I don't understand. Can you help me out here?

[calixtus]

Well, the context menu is still not perfect (some menu entries are still not created by factory methods, but with CustomMenuEntries to be able to add ToolTips to them, which is - as I see it - fairly uncommon). This results in misplaced labels:

I did some research but I see no way to preserve the ToolTips and to use standard MenuItems, so I'll leave that as it is. Everything else should be an improvement and using the command pattern this should also prepare some repairs in our broken undo/redo system.

[Siedlerchr]

Tested on mac + codewise also lgtm