A general mechanism is available to enable programmers to monitor the
window/control that has the cursor (or the focus, see ADD_FOCUS_MONITOR@).
CB_FUNC is the name of a routine that is either a function returning an
integer, or a SUBROUTINE. The return value (if any) is not used. Cursor
call-back functions can use the CLEARWIN_INFO@ parameter 'CURSOR_WINDOW' to
determine the window handle of the control or window that has the cursor.