WindowControl : move coordinate calculation From "AddClient" method to new "CalcChildRect" method
that allow to calculate before call "Create"

description of special value for coordinate :

Y :  use negative value to space down items
     -999 to stay on the same line
     -998 to advance one line with more space
     -997 to advance one line with twice more space

Width : use negative value for set right coordinate relative to Right coordinate of Parent Window