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]