gui: Recode syntax files on load and save.

gui: Recode syntax files on load and save.

This adds a new widget that allows an encoding to be selected to the
File|Open dialog box, as well as to the File|Save As... dialog box
for syntax files only.  Files are recoded to UTF-8 on load and back
to the original encoding on save.

File|Recent Files doesn't yet properly save the encoding information.
That's coming up in the next commit.

Bug #33254.
Thanks to John Darrington for reporting this problem.