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.
Renamed `MMCommonConnectProperties' to `MMSimpleConnectProperties', and removed
the `MMModemSimpleConnectProperties' provided in libmm-glib. We'll just use the
original one from libmm-common always.
