If we have an input parameter (or return value) we need to use (nullable).
However, if it is an (inout) or (out) parameter, (optional) is sufficient.
It looks like (nullable) could be used for everything according to the
Annotation documentation, but (optional) is more specific.
As it turns out, we have examples of internal functions called
type_name_get_private() in the wild (especially among older libraries),
so we need to use a name for the per-instance private data getter
function that hopefully won't conflict with anything.
Add GSimpleProxyResolver, for letting people do static proxy
resolution, and to use as a base class for other resolvers (such as
GProxyResolverGnome).
https://bugzilla.gnome.org/show_bug.cgi?id=691105
