Optionally use libnm-glib for NetworkManager integration

Optionally use libnm-glib for NetworkManager integration

Previously Empathy use the NM D-Bus interface directly. Unfortunately the d-bus
calls used were deprecated and somewhat buggy (if NM wasn't running on empathy
startup, it would never detect network changes).

Given that NetworkManager isn't a blessed gnome dependency the code has been
made optional, but it's expected that every distribution will build with
NetworkManager integration anyway.

Based on a patch by Tambet Ingo <tambet@novell.com>. Fixes #587446