SoupProxyResolverGNOME: make this a subclass of SoupProxyResolverDefault

SoupProxyResolverGNOME: make this a subclass of SoupProxyResolverDefault

modify SoupProxyResolverDefault so it's possible to override the
choice of g_proxy_resolver_get_default(), and make
SoupProxyResolverGNOME just be a subclass of SoupProxyResolverDefault
that specifically requests the "gnome" GProxyResolver.

libsoup-gnome no longer depends on libproxy and GConf. (Perhaps it
should now depend on gsettings-desktop-schemas, or else a
glib-networking that includes GProxyResolverGnome. But it will fall
back to using the libproxy-based proxy resolver in that case anyway,
so it's all good.)

https://bugzilla.gnome.org/show_bug.cgi?id=625898