Restore decibel scale option and fix some warnings

See my comment on https://github.com/Maato/volumeicon/commit/df1d6293817e.

I find the decibel scale more natural, especially on controls that have a wide dynamic range; for example, on my laptop, where I have only one 3.5 mm output, which I set to full volume for a line level device but to about -40 dB for headphones.