Cleanup after GStreamer actor
Cleanup after GStreamer actor

Unregister callbacks and release pipeline resources when GStreamer actor shuts
down.

Fixes #185.

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