Add an icon picker in group edit dialog for issue#6142

[EricLee543]

For issue #6142 , I add an icon picker in group edit dialog.
There are two ways to select an icon. First, select an icon by icon picker button. Second, input icon name in iconfield. This dialog is implemented by ScrollPane and Pagination. Here is a demo video.

• 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 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.

IconPIckerVideo.1.mov

[calixtus]

Hi @EricLee543 on first sight, this looks like a really cool idea, I did not yet look into the code though.
One idea to improve the UI in the group dialog: Maybe it is possible to unify the button icon picker and the text field into something combobox-like field and add a graphic node to the text displaying the selected icon in the text field.
Thanks for your work!

[Siedlerchr]

Please have a look at the checkstyle/reviewdog stuff

[koppor]

@ericlee Welcome to JabRef development.

The first thing to learn about open-source-projects is that developers have limited time resources. They rely on automatation.

Did you see our automated tools?

Following is accessible via: https://github.com/JabRef/jabref/pull/7758/files

Is this part of a university course? If yes, based on our long experiences, I see the following things: Your supervisor seem not to have contacted us in advanced (see https://devdocs.jabref.org/teaching for details). Thus, I have to ask you whether you intend to work on this PR for the next weeks?