For channels that have a defined dB range, use that instead of the raw
volume range when calculating the volume percent.
Also use the same logic as alsamixer when calculating the percent from
the dB values: assume a linear scale if the dB range is “small
enough”, and otherwise normalize it against a logarithmic scale.
With this, yambar’s “percent” value matches alsamixer’s exactly.
The ‘volume’ tag remains unchanged - it always reflects the raw volume
values.
Instead, we add a new tag ‘dB’, that reflects the dB values.
Closes#202
6c10eb2153