zeroconf: Fix intermittent dbus/avahi exception

This fixes an issue where I sometimes would get an error from dbus
'Unable to guess signature from an empty list'. After some digging
and checking the avahi dbus specs I found they expect the text list
to have a signature of 'aay' (an array of arrays containing bytes).
So instead of using python lists we now use a 'typed' dbus array.

It is not clear to me why this is a heisenbug, but this fix does
seem to make it go away.

(cherry picked from commit 80f5c9158d485099fac9693d7fbad74a29578a09)
docs: Update changelog
Merge branch 'release/v0.19.x' into develop
Fail early if extension doesn't implement setup()

Fixes #813
Merge branch 'release/v0.19.x' into develop
docs: Use text from web site and readme
docs: Add more links into the docs
docs: Use text from web site and readme

(cherry picked from commit 1e0569abb6e482366b6012e05604826a4df35c12)
docs: Add more links into the docs

(cherry picked from commit 5dd2be5ec94c8691eb1051e9e78c283a2e09e764)
docs: Update changelog
Bump version to 0.19.3
Release v0.19.3
Merge branch 'master' into develop

Conflicts:
	docs/changelog.rst
audio: Split out the signals tracking
audio: Add docs to signal handler helper
audio: Split out appsrc handling
audio: Use pbutils to provide usable plugin missing info
audio: Improve GStreamer logging

This adds an extra mopidy.audio.gst logger and moves the GStreamer logging to
it. Additionally this adds more logging so we can likely get by with just
mopidy logs in more cases.
audio: Split out ouput handling from audio actor

This also lays some basic ground work for handling multiple outputs.
audio: Unify source handlers

notify::source and setup-source are the same, just that setup-source is a
convenience wrapper.