Which requires that we turn it into a GInterface that MMModemBase
can implement, because dbus-glib does not allow attaching more
than one introspection glue structure to an object at a time.
Also implement the standard D-Bus properties changed signal.
* Add IpMethod property with known values ppp (default), static, DHCP.
* Rename DataDevice property to Device.
* Add GetIP4Config method. It should be implemented only when IpMethod==static.
* Update org.freedesktop.ModemManager.Modem.Gsm.Sms interface based on
Pablo Martí Gamboa's suggestions.
* Adjust MBM and HSO interfaces to take advantage of the generic Modem
interface.