Fine-grained volume control widget

[marscher]

First of all thank you so much for this great application.

I'd like to request a feature for the equalizer. It would be very nice for the usage of Bluetooth audio devices to have more fine grained control over the total volume. Therefore it'd be very useful to have an additional slider controlling the volume of all frequencies with one additional slider.

Maybe thats already possible with the postgain value, but I find it rather contra intuitive and entering a number instead of sliding makes it harder to adjust.

If you point me to the right direction of the code, I could also send a PR. Thanks for considering!