GSocketClient: fix handling of application proxies

g_socket_client_add_application_proxy() claimed "When the indicated
proxy protocol is returned by the #GProxyResolver, #GSocketClient will
consider this protocol as supported but will not try to find a #GProxy
instance to handle handshaking." But in fact, it did the checks in the
wrong order, so GProxy proxies ended up overriding
application-specified ones. Fix that.

Also, simplify the code a bit by making use of g_hash_table_add() and
g_hash_table_contains().

https://bugzilla.gnome.org/show_bug.cgi?id=733876