Add XfceConsolekit and XfceSystemd

It's about sharing code used by xfce4-session and xfce4-power-manager,
like XfceScreensaver in libxfce4ui!89. There were more differences
between the two sources this time though. Xfce4-session was preferred,
both because the API was more extensive and because the implementation
was better.

Related: libxfce4ui!89