core: add PrimaryConnectionType property to NMManager (bgo #739080)

core: add PrimaryConnectionType property to NMManager (bgo #739080)

This will provide an extremely easy way for applications to find out
what type of connection the system is currently using.  They might want
to do this to avoid using data if a phone is on a 3G connection, for
example.

Having this as a separate property provides at least two advantages:

 1) it reduces code complexity for those wanting only this one simple
    piece of information

 2) we could allow access to this property (but nothing else) to
    privilege-separated applications in the future

This patch adds the missing nm_active_connection_get_connection_type()
which was in the header file but never actually implemented.

https://bugzilla.gnome.org/show_bug.cgi?id=739080