Follow up to #9422

Got a bit messy and mixed up, because the getKeywordDelimiter was very thin spreaded throughout many parts of the code. Review is probably easiest commit by commit.

• Cleaned up some preference migrations
• Introduced preferences tab 'Entry' and reordered some preferences into it from 'General' (idea is to dissolve general tab eventually and reorder its contents into other preferences, like language into appearance etc.; collect clusters of preferences like entry preview+entry table+entry editor+editor tabs or entry+group+key generator, etc.)

Entry tab:

General tab:

• 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.