Rewrite the JS keyboard handling to cope with IE and Chrome.

Rewrite the JS keyboard handling to cope with IE and Chrome.

Unlike Firefox, IE and Chrome don't generate keypress events at all if
you suppress the default handling of keydowns. Therefore, we have to
figure out everything from the keydown event, because if we unsuppress
the default handling of any keydowns then we'll get annoyances like ^R
going back to meaning reload-page rather than redo-move.

[originally from svn r9810]