Support for systemd graphical-session.target

Starting an XFCE session doesn’t start graphical-session.target in the user systemd session. It would be nice if it did, since that’s the documented standard way to start user services only in a graphical session but not a console. Note that graphical-session.target has RefuseManualStart, so it can’t just be started directly; per the systemd.special man page, the intended way for this to work is, to quote:

A target for a particular session (e. g. gnome-session.target) starts and stops "graphical-session.target" with "BindsTo=graphical-session.target".

So I guess there should be an xfce4-session.target which does that, and can also depend on various XFCE-only things if desired.