Prune config action

[yktoo]

The indicator's config (stored as indicator-sound-switcher.json) contains all devices ever seen by the app, including those that are offline or don't exist anymore. Which makes sense, for example, your paired Bluetooth headphone can be off at times.

Sometimes it's desirable though to get rid of obsolete device configurations, this is where a Prune config button can come in handy.

Originates from #118.