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.

introspection/org.freedesktop.ModemManager1.Modem.xml

@@ -259,7 +259,9 @@
     <!--
         MaxActiveBearers:
 
-        The maximum number of active packet data bearers the modem supports.
+        The maximum number of active
+        <link linkend="MM-BEARER-TYPE-DEFAULT:CAPS"><constant>MM_BEARER_TYPE_DEFAULT</constant></link>
+        bearers that may be explicitly enabled by the user.
 
         POTS and CDMA2000-only devices support one active bearer, while GSM/UMTS
         and LTE-capable devices (including LTE/CDMA devices) typically support