[ticket/12509] Add an option to set the path of custom file for extensions

https://tracker.phpbb.com/browse/PHPBB3-12509

PHPBB3-12509
[ticket/12509] Update acp custom_style setter

PHPBB3-12509
[ticket/12509] Fix the call to set_custom_file in adm/index and messenger

PHPBB3-12509
[ticket/12509] Fix doc block

PHPBB3-12509
[ticket/12509] Fix typo in adm/index.php

PHPBB3-12509
[ticket/12509] Fix path in adm/index.php

PHPBB3-12509
[ticket/12509] Update calls to set_custom_style in forgotten places

PHPBB3-12509
[ticket/12509] Add missing ',' in arrays

PHPBB3-12509
[ticket/12509] Add example in doc block

PHPBB3-12509
Merge pull request #2455 from Nicofuma/ticket/12509

[ticket/12509] Add an option to set the path of custom style directory (ie: email templates) for extensions

* Nicofuma/ticket/12509:
  [ticket/12509] Add example in doc block
  [ticket/12509] Add missing ',' in arrays
  [ticket/12509] Update calls to set_custom_style in forgotten places
  [ticket/12509] Fix path in adm/index.php
  [ticket/12509] Fix typo in adm/index.php
  [ticket/12509] Fix doc block
  [ticket/12509] Fix the call to set_custom_file in adm/index and messenger
  [ticket/12509] Update acp custom_style setter
  [ticket/12509] Add an option to set the path of custom file for extensions