[ticket/11852] Split filesystem and path_helper into 2 classes

PHPBB3-11852
[ticket/11852] Move tests to folder with new class name

PHPBB3-11852
[ticket/11852] Add class file

PHPBB3-11852
Merge pull request #1735 from nickvergessen/ticket/11852

Ticket/11852 filesystem class must not depend on a web request