Pipewire support for Mixxx

Currently, Mixxx uses PortAudio as a high level abstraction over audio APIs across all supported platforms, which only exposes the common subset of all those APIs, missing Linux specific features and conventions. This implementation will add Pipewire audio API support to Mixxx for Linux, listen for PipeWire events to support soundcard hotplug and update routing UI according to changes made in an external patchbay. It will also allow Mixxx to route audio to and from any source/sink. This proposal also attempts to achieve ALSA comparable latency with the PipeWire backend.