Can't change sound card index at runtime

[dcpurton]

It's possible to change the card_index preference either in the preferences panel or in the config file, but this only takes affect after quitting and restarting Volti.

It would be good to be able to change which card is controlled at runtime. This would make it easier to switch to controlling the volume over HDMI.

(Volti 0.2.3 under Debian)