When a wlroots compositor runs as a systemd user unit there is no
session associated with the compositor process. Instead we need to
attach to an active and graphical user session.
This change first looks for an available session for the process, and if
there isn't one falls back to display in the oldest available graphical
session.
This work was modeled after a similar change to mutter -
https://gitlab.gnome.org/GNOME/mutter/merge_requests/150.
Sessions can now be retrieved from a backend in a more general manner.
Multi-backend gets back its `session` field that contains the session
if one was created, removing the interfacing from multi backend with the
drm backend directly. This adds the possibility to use sessions even
without the drm backend.
It additionally fixes the bug that 2 session objects got created when
WLR_BACKENDS were set to "libinput,drm".
To allow vt switching without drm backend (and drm fd) on logind, start
listening to PropertiesChanged signals from dbus and parse the session
"Active" property when no master fd was created (this does not change
current drm backend behaviour in any way).
