Bump minimum GLib requirement to 2.36.

Needed for GSimpleProxyResolver.
Remove all g_type_init() calls.

g_type_init() is deprecated in GLib 2.36.
EMailSession: Implement ref_proxy_resolver() method.

Eventually I'd like to add an ESourceExtension for proxy settings,
but for now we'll live with the old and somewhat broken EProxy API.