Add audio support to GStreamer backend

Signed-off-by: Piotr Galiszewski <pgaliszewski@corp.bluecherry.net>
Add some documentation to audio code

Signed-off-by: Piotr Galiszewski <pgaliszewski@corp.bluecherry.net>
Add initial version of volume control

Signed-off-by: Piotr Galiszewski <pgaliszewski@corp.bluecherry.net>
Fix unnecessary double video reseting when changing video event

Signed-off-by: Piotr Galiszewski <pgaliszewski@corp.bluecherry.net>
Remember audio settings between different videos

Signed-off-by: Piotr Galiszewski <pgaliszewski@corp.bluecherry.net>
Merge branch 'audio-support' of https://github.com/pgalisz/bluecherry-client

Signed-off-by: Piotr Galiszewski <pgaliszewski@corp.bluecherry.net>