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.

docs/reference/api/ModemManager-sections.txt

@@ -10,6 +10,7 @@ MM_CHECK_VERSION
 <SECTION>
 <FILE>mm-enums</FILE>
 <TITLE>Flags and Enumerations</TITLE>
+MMBearerType
 MMBearerIpFamily
 MMBearerIpMethod
 MMBearerAllowedAuth