Fix adjusting clocks in Xaw version

Fix adjusting clocks in Xaw version

The handler attached to Label options failed to flip the sign of the
option number when a button other than Button1 was pressed, so that time
would always be subtracted on Shift + ClockClick and never added.
Shift now also suppresses interpretation as null move when playing.
(In Analyze / EditGame mode it is still null move starting a variation.)