GSettingsBackend: add read_user_value() API

GSettingsBackend: add read_user_value() API

This will get the 'user' value from the database (ie: the one that the user has
control over).

Provide a default implementation that chains to ->read().  That will work for
all of our internal backends which don't have a concept of layering or
lockdown.

The delayed backend implments "user value" by returning anything that's
in the changeset (incuding an explicit NULL) or chaining up otherwise.

We will use this for g_settings_get_user_value().

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