is-enabled? in radio-box%
Returns #t if the window is enabled when all of its ancestors
are enabled, #f otherwise.
( -> booleansend a-radio-box is-enabled?)
Returns #f if the entire radio box is disabled,
#t otherwise.
( -> boolean
send a-radio-box is-enabled? n)
n : exact non-negative integer
Returns #f if nth radio button is disabled (independent of
disabling the entire radio box), #t otherwise. Radio buttons
are numbered from 0. If n is equal to or larger than the
number of radio buttons in the radio box, an exn:application:mismatch exception is raised.