Added icon picker in group dialog

[BasilDimopoulos]

Heya guys, I understand that this issue already has a pull request associated, however, we were just hoping to get some feedback on this for our university project. We have developed a UI selector that fills out the icon text field. Fixes #6142 .

• 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.

[calixtus]

Did you notice, @EricLee543 opened a similar PR #7758 some days ago?
One of the two PRs is probably lost work now...

[calixtus]

On first sight: There are several issues mentioned by the reviewdog and seeing your screenshot, something seems of with theming/style of the icon select dialog.

[koppor]

@BasilDimopoulos 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?

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?

[tobiasdiez]

For better performance, one could maybe use https://github.com/FXMisc/Flowless which should be able to handle a large amount of icons (because only a bunch of them are actually created).

Might also be helpful for the maintable actually, which would address @k3KAW8Pnf7mkmdSMPHz27's question!

[Siedlerchr]

Another suggestion would be to add an interface like this:
Search and display the icons by category (not sure if it's possible with kordamp)

https://icon-sets.iconify.design/mdi/

[calixtus]

[Siedlerchr]

Suggested change

	ikonList.addAll(allOf(MaterialDesignA.class));
	Set<IkonProvider<Ikon>> result = ServiceLoader.load(IkonProvider.class.getModule().getLayer(), IkonProvider.class).stream()
	.filter(provider -> provider.get().getClass() != JabrefIconProvider.class)
	.map(Provider::get).collect(Collectors.toSet());
	for (IkonProvider provider: result) {
	ikonList.addAll(allOf(provider.getIkon()));
	}

[calixtus]

Something seems to be odd. Works with eclipse, but not with gradlew / javac

                                                      .map(ServiceLoader.Provider::get).collect(Collectors.toSet());
                                                                                               ^
    (argument mismatch; Collector<IkonProvider<Ikon>,CAP#2,Set<IkonProvider<Ikon>>> cannot be converted to Collector<? super IkonProvider,CAP#2,Set<IkonProvider<Ikon>>>)
  where R,A,T#1,T#2 are type-variables:
    R extends Object declared in method <R,A>collect(Collector<? super T#1,A,R>)
    A extends Object declared in method <R,A>collect(Collector<? super T#1,A,R>)
    T#1 extends Object declared in interface Stream
    T#2 extends Object declared in method <T#2>toSet()
  where CAP#1,CAP#2 are fresh type-variables:
    CAP#1 extends Object from capture of ?
    CAP#2 extends Object from capture of ?```