Draft: backends/native: Register X11 display with logind
logind has the ability to export the X11 display a session uses with the logind session.
Previously, this mapping had to be set up at session opening time, so we lost the feature when we moved to starting the X server within the session instead of before the session.
Recently, logind gained a SetDisplay dbus method to allow the X11 display to be set after session open time.
This commit makes use of that new method.