audio: Make stream-changed correctly handle tee setup

For the purposes of this event we consider the outputs sink the final element.
If we don't do this we would get one event per branch, and we don't want to
track when each of them actually switches any way. So just tracking when the
tee/outputs bin gets the event is good enough for us.

As part of this I've also added 'testoutput' as a special cased output value.
This is now needed as outputs are always synced to the clock, making testing
a lot less practical.
audio: Move most of event handling out of audio.

Some of the signal handling still needs to be moved.
#818 Remove mopidy lux from documentation

(cherry picked from commit 06d7d650f870dfe55e02bc091f6f344df2762ef9)
(cherry picked from commit c230a0f64cf48eac18d72dc04827afe88220bcc6)
docs: Add sponsors page

(cherry picked from commit b40409141fe6b8012ff11bb2d9fb041831e02571)
local: Return multiple tracks from lookup()
Fixed flake8: I201 Missing newline between sections or imports
docs: Update changelog
Bump version to 0.19.4
Release v0.19.4
Merge branch 'release/v0.19.x' into develop

Conflicts:
	docs/changelog.rst
docs: Fix syntax error
Change JsonLibrary.lookup to return a list
docs: Add Discuss link to readme
docs: Rewrite 'getting help' section
docs: Fix syntax error

(cherry picked from commit cf5660e8e5b4be902d8697ea0f4843356efb9359)
docs: Add Discuss link to readme

(cherry picked from commit 61e724eb0e314e5ec5d8e39f2319a1d8a7e193c1)
docs: Rewrite 'getting help' section

(cherry picked from commit 13c92dae65ac86cea64e21009a02499801a7e40a)
Merge branch 'release/v0.19.x' into develop
audio: s/Triggering/Audio event/
audio: Resolve review comments