api,bearer: new 'BearerType' property

Until now we have only allowed to use and setup 'default bearers' (in
4G) or 'primary contexts' (in 2G/3G).

We can define a couple of additional bearer types, though:

* The 'dedicated bearers' (in 4G) or 'secondary contexts' (in 2G/3G),
  which are associated to a specific default/primary one, but which
  provide specific QoS settings configured via traffic flow templates.

* The 'initial default EPS bearer', which is a special case of default
  bearer in LTE, which is automatically created and connected when the
  modem is registered in the LTE network.

This commit introduces a new 'MMBearerType' enumeration that will be
associated to each bearer through a 'BearerType' property in the
org.freedesktop.ModemManager1.Bearer interface, showing what kind of
bearer/context this is.

By default, right now, all bearer objects created are 'default'
bearers.

cli/mmcli-output.c

@@ -184,6 +184,7 @@ static FieldInfo field_infos[] = {
     [MMC_F_LOCATION_CDMABS_LAT]             = { "modem.location.cdma-bs.latitude",       "latitude",              MMC_S_MODEM_LOCATION_CDMABS, },
     [MMC_F_FIRMWARE_LIST]                   = { "modem.firmware-list",                   "list",                  MMC_S_MODEM_FIRMWARE,        },
     [MMC_F_BEARER_GENERAL_DBUS_PATH]        = { "bearer.dbus-path",                      "dbus path",             MMC_S_BEARER_GENERAL,        },
+    [MMC_F_BEARER_GENERAL_TYPE]             = { "bearer.type",                           "type",                  MMC_S_BEARER_GENERAL,        },
     [MMC_F_BEARER_STATUS_CONNECTED]         = { "bearer.status.connected",               "connected",             MMC_S_BEARER_STATUS,         },
     [MMC_F_BEARER_STATUS_SUSPENDED]         = { "bearer.status.suspended",               "suspended",             MMC_S_BEARER_STATUS,         },
     [MMC_F_BEARER_STATUS_INTERFACE]         = { "bearer.status.interface",               "interface",             MMC_S_BEARER_STATUS,         },