Part of #6435.
Reset Preferences in the Preferences dialog now opens a question over
the dialog (`resetPreferences`). Cancel or Escape goes back to
Preferences as it was, with its edits, category and search kept. OK goes
back with every preference at its default value; OK in Preferences
applies them and Cancel there discards them, as before. `prefs.reset`
still resets at once.
Docs: docs/mcp.md, docs/target-app-parity.md.
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