Follow-up preview tab for diff file views

[fcollonval]

    Great work here, it is a nice feature to have!

• Could we add a setting to disable this behaviour? It seems that add a setting for only using a single tab for diffs #620 proposed adding this as an opt-in setting.
• When I move the diff to another spit panel (say to the right):
  • the italic style in tab bar disappears (probably the class does not stick after reattaching in another panel)
  • if I open another diff the old diff in right panel gets closed and reopened in the left panel. I find this annoying, I guess it was not intended.
• I had no idea what italic style means before looking at the docs; it would be helpful if there was an explanation in the title, or maybe even a pin icon; a pin icon could be implemented as unicode prefix (📌 or 📍or 🖈) added to title

Originally posted by @krassowski in #1184 (comment)