Allow sending a port message and receive a reply

Instead of manually matching a sent message to a port message, allow adding a serial which is then used to
relate a reply back so you just get a neat Promise-based API.
For compatibility, the existing method cannot be changed, as with new extension and older host I will never
get a reply, and I cannot tell whether the caller actually asked for the promise or not and if they didn't
we would leak the promise resolvers as we never got a reply.

Differential Revision: https://phabricator.kde.org/D23099