[feature/twig] Change style->set_style to accept a list of base directories

set_style now accepts an array containing a list of paths, e.g. array(
'ext/foo/bar/styles', 'styles'). Default: array('styles')

Using this option allows us to set the style based on the user's preferred
style (including the full tree), but use one or more base directories to
add the paths from.

The main use for this ability is so that extensions can call set_style,
including their path and the phpBB styles path (or any others) and have
their template files loaded from those directories (in the order given).

PHPBB3-11598
[feature/twig] Add set_style function to controller helper

PHPBB3-11598
[feature/twig] Call set_style in the foo/bar controller for functional tests

PHPBB3-11598