nmcompat: Support nm dbus properties

nmcompat: Support nm dbus properties

After some investigation I found that network-manager also provides
the network state through properties and libnm-glib library uses this
instead of the GetState() method.

Add Get() method call retrieve the property and a PropertyChanged signal
to notify state changes. Now empathy, which uses libnm-glib, properly
detects network state changes.

I deliberately omitted GetAll() method because libnm-glib doesn't seem
to need it.