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
GSettings: small internal refactor

Add two boolean parameters to our internal getter utility function in
anticipation of the coming addition of g_settings_get_user_value() and
g_settings_get_default_value() APIs.

https://bugzilla.gnome.org/show_bug.cgi?id=668233
GSettings: add getters for user/default value

Add two new APIs: g_settings_get_user_value() and
g_settings_get_default_value().   Together, these should allow the
inspection of all interesting cases of "is this key set?" and "what
would happen if I reset this key?"

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