audio: Update mixer track selection logic (fixes #307)
audio: Update mixer track selection logic (fixes #307)

We now ensure that the track we choose has one or more volume channels we can
control. This change also fixes that fact the MIXER_TRACK setting would not
work if we happened to find a track that was flaged as MASTER OUPUT before
finding the right label, so far no one has reported this as an issue.

dbrgn/mopidy,tkem/mopidy,swak/mopidy,glogiotatidis/mopidy,adamcik/mopidy,SuperStarPL/mopidy,ZenithDK/mopidy,mokieyue/mopidy,SuperStarPL/mopidy,rawdlite/mopidy,quartz55/mopidy,SuperStarPL/mopidy,hkariti/mopidy,jodal/mopidy,ali/mopidy,tkem/mopidy,rawdlite/mopidy,kingosticks/mopidy,quartz55/mopidy,kingosticks/mopidy,vrs01/mopidy,pacificIT/mopidy,dbrgn/mopidy,rawdlite/mopidy,liamw9534/mopidy,ZenithDK/mopidy,abarisain/mopidy,abarisain/mopidy,bencevans/mopidy,bacontext/mopidy,quartz55/mopidy,SuperStarPL/mopidy,priestd09/mopidy,pacificIT/mopidy,jmarsik/mopidy,ali/mopidy,vrs01/mopidy,liamw9534/mopidy,priestd09/mopidy,pacificIT/mopidy,jcass77/mopidy,jmarsik/mopidy,pacificIT/mopidy,hkariti/mopidy,jodal/mopidy,mokieyue/mopidy,mopidy/mopidy,woutervanwijk/mopidy,bencevans/mopidy,ZenithDK/mopidy,diandiankan/mopidy,swak/mopidy,ZenithDK/mopidy,kingosticks/mopidy,woutervanwijk/mopidy,tkem/mopidy,jcass77/mopidy,vrs01/mopidy,ali/mopidy,glogiotatidis/mopidy,ali/mopidy,mopidy/mopidy,bacontext/mopidy,dbrgn/mopidy,swak/mopidy,dbrgn/mopidy,jmarsik/mopidy,glogiotatidis/mopidy,jcass77/mopidy,diandiankan/mopidy,adamcik/mopidy,bencevans/mopidy,hkariti/mopidy,bacontext/mopidy,swak/mopidy,jodal/mopidy,jmarsik/mopidy,glogiotatidis/mopidy,bacontext/mopidy,diandiankan/mopidy,adamcik/mopidy,hkariti/mopidy,rawdlite/mopidy,quartz55/mopidy,priestd09/mopidy,bencevans/mopidy,mokieyue/mopidy,diandiankan/mopidy,tkem/mopidy,vrs01/mopidy,mopidy/mopidy,mokieyue/mopidy