GSettings: implement _read_user_value()

Implement g_settings_backend_read_user_value() in DConfSettingsBackend.

This will help us support g_settings_get_user_value() properly.

We add a new engine API to support this as well.  It takes a
read_through queue, even though we don't bother using that from
DConfSettingsBackend.

https://bugzilla.gnome.org/show_bug.cgi?id=668233