PulseAudio: Rework Volume

This intends to introduce some desktop feeling.
- When we start a new stream - it is not (!) unmuted. If user has muted it, we honor this until user wants to change volume.
- When user changes volume, we read in the external volume first and sync our internal volume to it, then user can press + or minus, cause he might have adjusted it with e.g. pavucontrol
- xbmc changes the stream's volume directly, you can see that in pavucontrol
- xbmc's volume state is synced, when next xbmc volume change happens

Edit: Splitted up into bugfixes and features with https://github.com/xbmc/xbmc/pull/4457