Add some javadoc and fix the preview update issue

[leitianjian]

Fixes #6447

I have taken a long time considering how to update the preview when preference change. After I found that the PreviewPanel will be bind to entry in FieldsEditorTab.java, then we can just update the layout in PreviewPanel.java#setEntry()

Some ideas:
Can we using observer pattern to refactor the code: JabRefPreferences as Observable object and component need preferences can register as Observer. If the preferences were changed, we can notify the affected component to do an update?

• 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.

[Siedlerchr]

Thanks, codewise looks good to me. We are currently in the process of refactoring the preferences in general and trying to solve the "updating of preferences" without restarting JabRef as well.

[leitianjian]

Thanks, codewise looks good to me. We are currently in the process of refactoring the preferences in general and trying to solve the "updating of preferences" without restarting JabRef as well.

What is the current idea of refactoring these preferences code? I wonder where I can see these discussions? Thanks!

[calixtus]

@leitianjian : developer discussion happens here: https://gitter.im/JabRef/jabref .
Feel free to participate or even contribute. If you like, we can talk a bit about the principles behind my current attempt in refactoring the preferences.

[leitianjian]

@leitianjian : developer discussion happens here: https://gitter.im/JabRef/jabref .
Feel free to participate or even contribute. If you like, we can talk a bit about the principles behind my current attempt in refactoring the preferences.

Thanks for your kindness. Recently I'm busy with other things. So, I cannot fully participate in it, but, I will keep track of your pr and the chat in gitter. Maybe I can learn something or provide some ideas

[Siedlerchr]

Thanks again for your contribution! I took the freedom to merge it ;)

[leitianjian]

Thanks!