n is an optional positive integer which is used to specify the width of
grey_ctrl is set to 1 to enable the button and 0 to disable (grey) it.
button_text is a standard character string (can be replaced by @) and can
include an ampersand (&) to mark an accelerator key. To change the text
dynamically use %lc - Handle of Last Control and change_button_text@.
The background colour can be specified using %bc and the text font and size can
be specified using %fn - Font and %ts - Text Size. Other effects can be obtained by using %lc - Handle of Last Control.