Clean up g_settings_list_schemas()

In its previous form, g_settings_list_schemas() was not useful as a tool
to prevent aborts due to using g_settings_new() with an invalid schema
name.  This is because g_settings_list_scheams() also listed relocatable
schemas, and calling g_settings_new() for those would abort just the
same as if you called it for a non-existent schema.

Modify g_settings_list_schemas() so that it only returns schemas for
which it is safe to call g_settings_new().  Add another call for sake of
completeness: g_settings_list_relocatable_schemas().

gio/gsettings.h

@@ -71,6 +71,7 @@ struct _GSettings
 GType                   g_settings_get_type                             (void);
 
 const gchar * const *   g_settings_list_schemas                         (void);
+const gchar * const *   g_settings_list_relocatable_schemas             (void);
 GSettings *             g_settings_new                                  (const gchar        *schema);
 GSettings *             g_settings_new_with_path                        (const gchar        *schema,
                                                                          const gchar        *path);