Use sound theme for volume feedback and speaker test

Read the sound theme setting and use it to play the volume feedback and speaker test sounds. If the theme do not provide them, it will fall back to the previous 'freedesktop' sounds.

On the volume feedback case we want to keep the sound in the permanent cache to reduce the latency ("permanent" option), but invalidate it when the theme changes.

Latest part of plasma-workspace#40 (closed).