Jonas Koelker points out that the backspace key didn't work in GTK

Guess, because Guess expected ^H whereas GTK generated ^?. Other
puzzles that use Backspace do it by being prepared to see either,
which seems wasteful. Now the midend normalises both into ^H, so
front ends can generate whichever they like while puzzles can
safely just look for ^H.

[originally from svn r8786]

midend.c

@@ -752,6 +752,15 @@ int midend_process_key(midend *me, int x, int y, int button)
     if (button == ' ')
       button = CURSOR_SELECT2;
 
+    /*
+     * Normalise both backspace characters (8 and 127) to \b. Easier
+     * to do this once, here, than to require all front ends to
+     * carefully generate the same one - now each front end can
+     * generate whichever is easiest.
+     */
+    if (button == '\177')
+	button = '\b';
+
     /*
      * Now send on the event we originally received.
      */