session and seat management #897

The current documentation for session and seat management is pretty neutral and doesn't suggest any solution. It provides 4 sections with different options, where the user has to basically always choose dbus and then one or two other options, of which some overlap or conflict.

I think the core issues that should be addressed are:

1. dbus session bus: turnstile
2. XDG_RUNTIME_DIR: turnstile or elogind
3. user services for pipewire, pipewire-pulse and wireplumber: turnstile
4. Wayland: elogind or seatd
5. polkit, rtkit: elogind
6. power management (hooks, events and triggering (swayidle, gnome, kde)): elogind

If we want to solve them that makes turnstile a hard requirement, no more hacky dbus-run-session, no more xdg autostart bullshit for user services.

Then the next choice is between elogind and seatd, seatd only provides the bare minimum to make wayland work. elogind solves the core issues and provides additional features like uaccess udev device access and the logind dbus api for desktop environments.

Might also make sense to just make this default for the desktop ISOs.