Expose message delivery state changes to websockets API

We already indicate whether a message was sent/acked, but we don't
inform about updates.

Needed for briar-gtk#69.

Fixes #1779
1 job for 1779-headless-messages-sent-acked in 9 minutes and 2 seconds (queued for 8 minutes and 32 seconds)
Status Job ID Name Coverage
  Test
passed #6140
test

00:09:02