Preferences ui cleanup

[calixtus]

• introduced web search tab
  
• renamed Import and Export tab to Export tab
  
• moved remaining content to entry tab, refined ui (reduced vertical space, added indentation)
  
• removed files tab
• removed unused code (TagBar / Tag, DiffHighlightingTextPane)

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1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. 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.

[koppor]

Reorganization also relates to #9836.

[koppor]

We have "Always reformat BIB file on save and export". Should we a) keep this setting both in "Library"/"File"/"General" and in "Export" or should we b) split it into "Always reformat BIB file on save" and "Always reformat BIB file on export". I lean to the former. But then, the UI setting appears in two different tabs. Hopefully, they are synchronized!

[calixtus]

I removed the files tab. So the autosave setting should not appear twice.

[Siedlerchr]

Oh no please not, my other PR is using the files tab

[calixtus]

Should not be a big deal to fix the merge conflicts

[koppor]

I removed the files tab. So the autosave setting should not appear twice.

It is about reformatting, not saving. 😅

Saving is only concerning the library, but reformatting both the library (.bib) and export (as .bib).

[calixtus]

I will make a follow up PR introducing a library tab. But I think it will be too much for this PR as I already changed 42 files with this one.

[calixtus]

Thx for #