xdg-shell: Add support for implicit popup repositioning

This commit adds protocol additions making it possible to implicitly
reposition an already mapped popup if the conditions for the constraint
changed (e.g. toplevel moved).

Implicit popup moving is done by setting a adjustment flag on the
positioner used to create it that will cause the compositor to adjust
the position as the conditions used to constrain it change.

These changes may include, for example, changes in the position of the
parent window or the geometry of the work area. To allow the client to
update its content in response to the updated position, the client must
ack the configure event, optionally with new content. Until the client
acks this configure event, the existing positioner will continue to be
used.

Implicit repositioning by itself is racy regarding inter-surface
synchronization of applied state. Inter-surface synchronization is
deliberately left out of xdg-shell, and left to be handled externally.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>