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

docs/reference/gio/gio-sections.txt

@@ -2442,6 +2442,8 @@ g_settings_revert
 g_settings_get_has_unapplied
 g_settings_get_child
 g_settings_reset
+g_settings_get_user_value
+g_settings_get_default_value
 
 <SUBSECTION Introspection>
 g_settings_list_schemas