Pipewire support

This is somewhat related to/partly duplicates #2:

Supporting Pipewire, potentially as fallback method, would allow Megapixels to work in a couple of cases where it does not right now - and could help iron out API questions on the Pipewire side. It would allow to work in sandboxed environments (using the xdg-camera-portal) and could benefit from the Pipewire libcamera backend, which works on a couple of devices, e.g. the Pinebook Pro (and support for the Pinephone Pro is close as well), with many more devices expected.

A big argument in favor of it would be that Pipewire will likely become the common way to access cameras, as e.g. webrtc (chromium/electron/firefox) has patches in development to support it. So Megapixels should arguably support it as well :)