[WIP][ticket/12775] Add a container_factory class and remove functions_container

https://tracker.phpbb.com/browse/PHPBB3-12775
https://tracker.phpbb.com/browse/PHPBB3-12420
https://tracker.phpbb.com/browse/PHPBB3-12774

PHPBB3-12775
What happens if config file contains $defined_vars? I suggested to use a property instead of a variable.