Move warning text about bare filenames

This takes this bit of text out of the actual window layout and moves it
to a dialog box that is only displayed if the user makes the correct
combination of options.
Only display warning on explicit behavior changes

This ceases calling this member function when the handling changes,
which will bother the user.
Automatically re-check behavior on handling change

This probably makes sense anyway, since we are changing the label and
want it to go back to the "default".