[ticket/13370] Allow calling class method in convertor framework directly.

This change allows the usage of array($class, 'method')) directly instead of
having to use the functionX syntax as follows:
array('function1' => array($class, 'method')))

PHPBB3-13370
Merge pull request #3223 from prototech/ticket/13370

[ticket/13370] Allow calling class method in convertor framework directly