INT 50 - Vanderaart TEXT WINDOWS, PC Thuis Shell - OPEN TEXT WINDOW
        AX = 0000h
        ES:BX -> name string or ES:0000h if none
        CH,CL = row,column of upper left corner
        DH,DL = row,column of lower right corner
Return: AX = window handle or
            0000h if not installed
            FFFFh on error
SeeAlso: AX=0002h"TEXT WINDOWS"