Bump minimum GLib requirement to 2.36.

Needed for g_socket_client_set_proxy_resolver().
Remove all g_type_init() calls.

g_type_init() is deprecated in GLib 2.36.
Remove other GLib 2.35.x checks.
Add camel_session_ref_proxy_resolver().

Replaces camel_session_get_socks_proxy().

Returns a GProxyResolver for the given CamelService.

camel_network_connect_sync() now calls this and hands the returned
GProxyResolver off to its GSocketClient just prior to connecting.