* 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.
Rework the MMCallbackInfo callback invocation.
Always use g_error_literal() where it makes sense.
Replace sleep() calls, with timeouts to not block the whole MM.