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.h

@@ -197,6 +197,7 @@ typedef enum {
     MMC_F_FIRMWARE_LIST,
     /* Bearer general section */
     MMC_F_BEARER_GENERAL_DBUS_PATH,
+    MMC_F_BEARER_GENERAL_TYPE,
     /* Bearer status section */
     MMC_F_BEARER_STATUS_CONNECTED,
     MMC_F_BEARER_STATUS_SUSPENDED,