xdg-shell/positioner: Replace edge bitfield with extended enum

Bitfields allowed for impossible combinations of anchor edges, such as
being on the left and right edge. Use of explicit enumerations means we
don't need to handle that case.

Signed-off-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>