aout: propagate volume and mute to input manager (fix #6760)
aout: propagate volume and mute to input manager (fix #6760)

Several of UI & control plugins follow the volume and mute status
through the playlist instead of the aout. This patch works around
that limitation.

krichter722/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc,shyamalschandra/vlc,vlc-mirror/vlc-2.1,krichter722/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,vlc-mirror/vlc,vlc-mirror/vlc-2.1,shyamalschandra/vlc,vlc-mirror/vlc-2.1,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,vlc-mirror/vlc,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,krichter722/vlc,jomanmuk/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,vlc-mirror/vlc,xkfz007/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,xkfz007/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,vlc-mirror/vlc-2.1,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2