Sound applet: do a full rescan when a sound card is added or removed

When adding a sound card, associated sinks weren't appearing in the "Output device..." menu because nothing was connected to the card-added signal.

This commit plugs card-added and card-removed to the same function as state-changed.
Merge pull request #2264 from deubeuliou/patch-1

Sound applet: do a full rescan when a sound card is added or removed