Show xfce4-session logout dialog on close-window shortcut with no window focus

When no window has focus, and you press the close-window keyboard shortcut, xfwm4 will contact xfce4-session and ask it to put up the logout dialog.

xfwm4 uses libSM to ask for a logout, but xfwl4 instead should use xfce4-session's dbus interface.