config.h should be included from every .c file, and it should be
included before any other include. Fix that.
(As a side effect of how I did this, this also changes us to
consistently use "config.h" rather than <config.h>. To the extent that
it matters [which is not much], quotes are more correct anyway, since
we're talking about a file in our own build tree, not a system
include.)
Rename NMVpnPluginUiInterface to NMVpnEditorPlugin (to clarify that
it's unrelated to NMVpnPlugin), and add it to NetworkManager.h.
Rename NMVpnPluginUiWidgetInterface to NMVpnEditor, because it's not a
widget, and will soon be used for non-gui editing too. (Also, add a
placeholder for the method that non-gui editing will use.)
Fix the typedefs to not mix up the (dummy) NMVpnEditorPlugin and
NMVpnEditor types with the types of their interface structs. Update to
use G_DEFINE_INTERFACE.
Drop NMVpnPluginUiInterfaceProp; it doesn't matter what codes plugin
implementations use for the interface properties that they implement.
