Since r6711, puzzles built with Gtk 1.2 would take double actions when a menu
accelerator key (N/U/R/Q) was pressed -- once for the menu accelerator, and
once more in key_event().
This workaround, while unlovely, should at least not break in future (since the
things it relies on are unlikely to change).

[originally from svn r6745]
[r6711 == 077aa510c78f3273bd0d4ca4f1ca14780822ebf9]