prefs-dialog: Expose hidden setting 'misc-highlight-urls'

This will avoid confusion between OSC 8 escape sequence hyperlinks and "classic hyperlinks". You'd think at first sight that there could be just one setting for both types of link, but on second thought two separate settings seems legitimate. 'misc-highlight-urls' has been around for a long time anyway, so I suppose it's best to keep both.

Closes: #362 (closed)