[Engine] Provide an upgrade path to the auto-recode feature.

If a user's encoding is UTF-8 (either explicitly or as a system default), switch
to ISO-8859-1 with recoding. Otherwise, only turn recoding on.
[Frontend-GNOME] Fix preferences related to UTF-8 recoding.

Store the UTF-8 recoding preference correctly and hide the checkbox if the
advanced options are off.
Merge pull request #56 from RavuAlHemio/experiments/utf8_upgrade_path