xdg-shell: add enums for tiled window state to toplevel configure

this adds implementation from a related discussion long ago in which
it was decided that it would be useful for clients to know if/where their
windows were tiled so that various behaviors and visuals could be modified
to improve UX

a window which is e.g., tiled on the right side of the screen would set the
right|top|bottom tiled states in configure

Signed-off-by: Mike Blumenkrantz <zmike@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
[Jonas: Minor formatting fixes]
Signed-off-by: Jonas Ådahl <jadahl@gmail.com>

Changes since v2: simplified docs
Changes since v1: added since=2 to enum members