INT 15 - TopView - SEND MESSAGE - "SETESC" - SET ESCAPE ROUTINE ADDRESS
        AH = 12h
        BH = 14h
        BL = message modifier
            00h handle is DWORD on top of stack
            01h define user stream
            04h intercept keystrokes from KEYBOARD to a window (handle on stack)
            05h intercept keystrokes from task's default KEYBOARD to a window
        STACK: (if window)   DWORD user stream number (14h-1Fh)
                             DWORD address of FAR user stream handler
               (if keyboard) DWORD address of FAR filter function

The keyboard filter function is called when the keyboard is in field mode.  On
entry,
        AL = character
        AH = 00h or extended ASCII code if AL = 00h
        BL = field number
        CH = cursor column
        CL = cursor row
        DL = field type modifier (sixth item in field table entry)
        DH = seventh item in field table entry
        ES:SI = window's handle
        DS:DI -> field table entry for field containing the cursor
The filter function should return
        AH = 00h use keystroke
             01h ignore keystroke
             FFh beep and ignore keystroke
Note: the filter function is not allowed to make INT 15, DOS, or BIOS calls