[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