Setting MUIA_Window_RefWindow to another MUI window
object will make the left and top position relative
to this reference window. Using the
MUIA_Window_Left(Top)Edge_Centered tag, you can easily
open one window within another.
Note that if your window has an id, the window will
remember its last position and reopen there. Thus,
this tag is only useful if you omit MUIA_Window_ID,
maybe for some small requester windows.