13 Switchable Button
and icon and a string label. It toggles between
display of just the icon and a display with the
label and the icon side-by-side.
is called when the button
is pressed. The string
used as discussed above.
If alternate-bitmap is supplied, then it is used
when the button is switched to the view that just shows the bitmap.
If it is not supplied, both modes show the same bitmap.
If the vertical-tight? argument is #t, then the button takes up
as little as possible vertical space.
Sets the visibility of the string part of the label.
Calls the button’s callback function.