xdg-shell: ack_configure must be strictly monotonic

Clients must send ack_configure in a strictly monotonic order wrt
received configure events. It is an error to send an ack_configure
request for a configure event which was sent prior to the last
ack_configure for that surface, or to send multiple ack_configures for
the same configure event.

Weston and wlroots already use this interpretation, however Mutter and
KWayland are more lax and allow duplicates. This clarification tightens
the spec working to explicitly encode the Weston/wlroots behaviour.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Closes: #21