W is the handle of a button control obtained using %lc.
The size of the button depends on the initial text supplied using
%bt or %tt. If BUTTONTEXT is longer than this then it
may not fit on to the button.
This facility should be used sparingly because a window with several
buttons that change their meaning on the fly may confuse the user.
One potential use is to create toggle buttons with text such as Start
and Stop. In this case you would typically place the call to
CHANGE_BUTTON_TEXT@ inside the callback routine for the button and use
a variable to keep track of state of the toggle.