[ticket/10844] Add phpbb_root_path to phpbb_style_resource_locator construct()

The phpbb_root_path needs to be removed from the style path, as the finder
already prepends it, and therefor is unable to find style files when the root
path is not ./

PHPBB3-10844
[ticket/10844] Add phpbb_root_path argument to resource_locator constructs

PHPBB3-10844
[ticket/10844] Remove phpbb_root_path from set_custom_style()

The root path is not part of the style name and is prepended to the file
search by the finder and therefor must be removed.

PHPBB3-10844