mpris: Only import 'indicate' if $DISPLAY is set
mpris: Only import 'indicate' if $DISPLAY is set

This avoids printing of the following error message:

    ImportError: could not import gtk.gdk

Fixes #311

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