An old branch I had lying around. This fixes our use of GTypeInterface. IIRC this was a prelude to something else I was going to hack on, but I never did the second part and don't remember what it was :)

I also renamed NMSystemConfigInterface to NMSettingsPlugin because (a) it's a better name, and (b) neither "NMSystemConfig" nor "NMSystemConfigInterfaceInterface" made sense as a name when splitting the type into separate object and class types.