Add method to watch connection state of contacts

Related to:
* briar-gtk#19
* briar!1260
6 jobs for watch-contact-state in 1 minute and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #5580
pycodestyle

00:00:31

passed #5581
pylint

00:00:34

passed #5582
pytest

00:00:32

93.0%
canceled #5576
pycodestyle

00:00:16

canceled #5577
pylint

canceled #5578
pytest