Support both GDM and LightDM
Old versions of xfce4-whiskermenu-plugin called gdmflexiserver, then it was changed to dm-tool.
But LightDM's dm-tool does not work under GDM:
$ dm-tool switch-to-greeter
Not running inside a display manager, XDG_SEAT_PATH not defined
Let's support both GDM and LightDM out of the box.
P.S. Maybe codestyle of my code is bad, feel free to suggest changes.