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 8 minutes and 10 seconds (queued for 7 minutes and 24 seconds)
Status Job ID Name Coverage
  Test
failed #6069
test

00:08:10

 
Name Stage Failure
failed
test Test

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BUILD FAILED in 6m 40s
137 actionable tasks: 116 executed, 21 up-to-date
Running after script...
$ rm -f $GRADLE_USER_HOME/caches/modules-2/modules-2.lock
$ rm -fr $GRADLE_USER_HOME/caches/*/plugin-resolution/
ERROR: Job failed: exit code 1