Add volume slider option

[ValentinMumble]

Hi!
Thanks for this wonderful app!
It would be great to have an option to display the Spotify's volume slider, I use Spotify on an external device using Spotify Connect and so the volume setting is different from my computer's one.

Keep up the great work!

[kmikiy]

do you have any idea, preference or suggestion where the volume slider should be in the UI and what it should look like?

[ValentinMumble]

I'm guessing either at the top or bottom, something like this?

Or outside of the cover if that's possible?

Anyway I would see if as an option disabled by default so that it does not overcharge the UI

WDYT?