PipeWire 0.3 and Plasma 5.20

[Aleix Pol]

Dear packagers,
For Plasma 5.20 we have added a (for now optional) dependency to
Pipewire 0.3. It is used for sending videos from the Wayland
compositor to the clients.

To have such a feature we need PipeWire 0.3 to be present. In fact, as
we developed the feature we found some issues upstream, so I would
recommend getting the latest stable release (0.3.7 right now, tested).

Alternatively screen/window casting won't be functional on Wayland,
including task manager thumbnails and screen sharing with the
different systems.

Best regards,
Aleix