Write string to window

        Entry:
          AH = 12h
          BH = 05h
          BL = object
              00h DWORD on top of stack is window handle
              01h write string to task's default window
          STACK: DWORD object handle if handle passed on stack
                 DWORD total length of string (high word == 0)
                 DWORD address of string to display

        Note: service routine will pop stack

        Return:
          Indicated actions performed
          A. non-control characters are displayed
          B. CR/LF/BS/Tab cause the usual cursor movement
          C. ESC starts a data structure with additional commands

        Data Structure:

        MAGIC  DB  1Bh
        MODE   DB  ?   ; 00h, 01h, 10h, 14h-1Fh legal
        LENGTH DW  ?   ; length of remainder in bytes
        var-length fields follow, each an OPCODE followed by
             zero or more args