Update the docs build to include and exclude the correct files.
Fill in some missing documentation, and fix problems in the existing
docs. (In particular, "<" can't appear as a literal in documentation,
so change it to "<". Also, "PKCS#12" has to be written as
"PKCS#<!-- -->12", or gtk-doc will think "#12" is a reference to a
type named "12".)
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.
