gio: Add g_async_result_legacy_propagate_error()

gio: Add g_async_result_legacy_propagate_error()

Finish deprecating the "handle GSimpleAsyncResult errors in the
wrapper function" idiom (and protect against future GSimpleAsyncResult
deprecation warnings) by adding a "legacy" GAsyncResult method
to do it in those classes/methods where it had been traditionally
done.

(This applies only to wrapper methods; in cases where an _async
vmethod explicitly uses GSimpleAsyncResult, its corresponding _finish
vmethod still uses g_simple_async_result_propagate_error.)

https://bugzilla.gnome.org/show_bug.cgi?id=667375
https://bugzilla.gnome.org/show_bug.cgi?id=661767