Changes the position of this in the Z-order (stacking order), so that it is above
above is true) or below
sibling is null, then this either raises (if
true) or lowers the window.
If this is a toplevel, the window manager may choose to deny the request to move the window in the Z-order, restack only requests the restack, does not guarantee it.
a Window that is a sibling of this, or null