Support for volume bar on dB scale

[tiagofreire-pt]

The dB scale has a relation between steps (000 to 185), as:

dB(STEP) = 0.5 * STEP - 80.5 [dB] or STEP(dB) = 2 * dB + 161 [steps]

I'm interested on having the volume bar with dB scale instead of steps one. This could also be configured on the yaml with something similiar with:

media_player:
- platform: asyncpioneer
  host: 192.168.8.121
  port: 8102
  volume_scale: 'dB'
# OR
  volume_scale: 'steps'

This feature is supported and displayed by the VSX-924, as the dB scale is limited between:

000VL == "--- min" 001VL == -80,5dB 002VL == -80 dB 185VL == +12 dB