Allow placing subsurfaces below parent surface

According to the spec, if the parent surface is specified in the
wl_subsurface.place_below(), the subsurface has to be rendered below the
parent surface.

At the moment, kwaylandserver doesn't handle that case properly. It is
not possible for sub-surfaces to go below the parent surface.

Another issue is that we wrongly assume that the place_above request
will put the subsurface on top of the stack if the parent surface is
specified as sibling. It doesn't seem like that's the case, not
according to the spec.

This change splits the child sub-surface list in two lists - below and
above. The alternative solution is to store the parent surface in the
children list, but it's an error prone solution and it's conceptually
weird.