api,dbus: 'ip-type' property now given as a MMBearerIpFamily (u)

Instead of using a predefined set of string values for 'ip-type' in
Modem.CreateBearer() and Simple.Connect(), we'll use an enumeration. The
implementation will then need to convert the requested IP family type to e.g.
the correct PDP type in 3GPP modems.

This change also consolidates the use of enums in dictionary properties when
possible to do so, as with the Rm Protocol.

docs/reference/api/ModemManager-sections.txt

@@ -1,6 +1,7 @@
 <SECTION>
 <FILE>mm-enums</FILE>
 <TITLE>Flags and Enumerations</TITLE>
+MMBearerIpFamily
 MMBearerIpMethod
 MMModem3gppFacility
 MMModem3gppNetworkAvailability