Wired up a "mixer-name" property.

Commented out lines demonstrate the issue with libintl setup:
- compilation warning (implicit _() definition)
- runtime crash (no '_' symbol)

Forgot to add them in the previous commit.

enable-keyboard-shortcuts property not yet working (?)

Icons audio-volume-...-panel used in sound-indicator are one size only.

Button with an icon reacting to volume changes and to mouse wheel events.