fltk: Handle '_' in button labels as keyboard shortcuts.

* fltk/main.cxx (convert_accel_utf8): New method.
(fltk_cmd_handler): Calls previous method to escape button labels.

• GnuPG-bug-id: T4336
• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>
• Co-authored-by: Michael Bäuerle