[ticket/11927] Correctly add new files on update

Currently we ignore language and style files when the directory where they
go to do not exist. However in 3.1 we introduce some new sub directories:
* language/en/email/short/
* styles/prosilver/theme/en/
So we need to change our check to look whether the language or style exist,
rather then the parent directory.

PHPBB3-11927
[ticket/11927] Add doc block to new function

PHPBB3-11927
[ticket/11927] Prefix function with phpbb_

PHPBB3-11927
Merge pull request #1853 from nickvergessen/ticket/11927

[ticket/11927] Correctly add new files on update