Allow game backends to use even special keystrokes such as N and Q;
they will only be processed as special by the midend if unwanted by
the backend. This causes 5x5 Solo to become just about playable,
because you can now click in a square and type `n'. However, typing
`n' when a square is not selected will revert to the normal
behaviour of starting a new game.

(This isn't particularly ideal, I know, but it's better than
nothing.)

[originally from svn r6048]