Design for Flatpak "device" portal

[Bastien Nocera]

Hey,

For a number of different programs, we might want to access external
devices. This mail is focused on making it possible to access raw USB
devices, but should be possible to adapt to other device types.

I added a comment to:
https://github.com/flatpak/xdg-desktop-portal/issues/227#issuecomment-426583435

Let me know whether anything is unclear.

It would be great if applications that want USB access could check
their sources for the way they do their filtering, and whether they
need access to USB interfaces other than the ones they initially
filtered for.

It would also be good to know what those apps use for device discovery
(udev? libusb's hotplug support?).

Cheers