Pulseaudio: More Desktop integration

This makes the volume slider in xbmc moving when it is controlled via pavucontrol. 

We are informed by a pulse callback and adjust the internal volume. then a message for the App Messenger is prepared. When application calls us to set the volume - we keep the set volume so that only the volumeBar is presented. In order to not break pulse amplification we ignore higher than 1.0 values.

The AppMessenger needs a straight forward adjustment in order to handle and forward the message for us.