Add mute button functionality

[tiberiuiurco]

I often need to do my ankis when other people are around and can't have the volume turned on. Or I would listen to music while doing my ankis and would like to only hear the music playing.
For that, I was always setting the volume to 0 but found it to frustrating to do it every time.
I made a mute button that does that and when unmuted, it preserves the previously used volume.