gio: add g_async_result_is_tagged()

Rather than doing a two step first-check-the-GAsyncResult-subtype-then-
check-the-tag, add a GAsyncResult-level method so that you can do them
both at once, simplifying the code for "short-circuit" async return
values where the vmethod never gets called.

https://bugzilla.gnome.org/show_bug.cgi?id=661767

docs/reference/gio/gio-sections.txt

@@ -1199,6 +1199,7 @@ GAsyncResultIface
 GAsyncReadyCallback
 g_async_result_get_user_data
 g_async_result_get_source_object
+g_async_result_is_tagged
 g_async_result_legacy_propagate_error
 <SUBSECTION Standard>
 G_ASYNC_RESULT