gtk: Disable echoing if backspace is pressed first.
* gtk+-2/pinentry-gtk-2.c (backspace_handler): New callback. (create_window): Attach previous callback to "backspace" signal.
- GnuPG-bug-id: T3428
- Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>