In WFD almost all rates default to 48kHz. So, to avoid re-sampling, set
the rate on the sink to 48kHz.

For now this adds a ScreencastPulseaudio class which creates a
gnome_screencast NULL sink and allows gettign a pipewire source for the
monitor source.

The NULL sink is currently not destroyed on shutdown, however, we do
make sure to only ever create one of them.