Rename "fallback" profile to Built-in

* Rename everything related to built-in profile both in code and UI.

* Unified style for [Read-only] and [Default] badges in profile
  manager's list model.

* Change --fallback-profile option to --builtin-profile.

* Backward compatibility: yes. If a user happened to name their
  profile "Built-in", it would continue to work as normal, and even
  load with `--profile "Built-in"` command line flag. It will let them
  modify any property including Name; but changing the name back
  to "Built-in" shows a warning and reverts the change as usual.

* Remove "This option is a shortcut for" sentence. While it is still
  technically possible to pass built-in profile's magic path, this
  option is not a shortcut, nor implemented as such.

* Delete extra naming conditions in ProfileManager::changeProfile,
  because they could never be triggered anyway, due to pre-flight
  checks in EditProfileDialog::isProfileNameValid. Automatic unique
  profile names generation has been done even earlier in either
  SessionController or ProfileSettings. Just as before, users will
  continue experiencing a generic "A profile with the name \"%1\"
  already exists." message.

Tests:

* Improve test for uncreatable file name of built-in profile.

* Add backward compatibility test for loading existing profile
  named "Built-in" which also references real built-in as its parent.

BUG: 438309