Clean up g_settings_list_schemas()

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().