Wishlist: Support for routing Pre-Fader-Level (PFL) deck audio to external devices

[fwcd]

Feature Description

For external (hardware) mixers it would be convenient if Mixxx provided the ability to route the individual decks before volume faders are applied, so the software faders could still be used e.g. for tracking the active deck while the actual attenuation happens in hardware. To avoid breaking anyone's audio setup, we should make this an option (perhaps even per-deck?), i.e. whether to export the PFL or the attenuated signal (as we currently do).

See the Zulip discussion for more info.