application: Export FileManager1 iface from dbus_register vfunc
The org/freedesktop/FileManager1 interface is not currently exported
from the dbus_register vfunc. This causes issues for projects (e.g.
desktop-icons extension) that want to use all the Nautilus intefaces
over the org.gnome.Nautilus connection. Let's use the already established
connection and export the FileManager1 interface from the dbus_register
vfunc.