User session management

[ardera]

instead of unconditionally grabbing access to KMS & input devices, we could talk to a session manager and implement session switching (using Ctrl+Alt+F3 for example), so people can test flutter-pi even on a device with a desktop environment.

There's a bunch of seat/session managers out there:

• systemd-logind
  • most commonly used manager nowadays, but you have to talk to it using DBus and the API is pretty big: https://www.freedesktop.org/software/systemd/man/org.freedesktop.login1.html
• elogind
  • same API as systemd-logind I think
• ConsoleKit
  • not actively maintained anymore
• seatd
  • not that widespread right now, I think
• old-school VT signals (not really a session manager, but technically that'd be sufficient)
  • implementation example, without the actual switching though: https://github.com/krh/vkcube/blob/f77395324a3297b2b6ffd7bce0383073e4670190/main.c#L479-L525

libseat looks kinda nice: https://git.sr.ht/~kennylevinsen/seatd

It's like a wrapper around all of them, though it's not yet available in debian bullseye, so I'd have to fetch & build it manually in the CMake file; but that's not really problematic I think.

[ardera]

@jwinarske Just pushed initial session switching support to the feature/compositor-ng branch. Needs libseat, so either you must have fairly recent ubuntu or you need to build it yourself locally

Still need to make sure I got all the implementation details correct though