[ticket/12346] Add Template Event overall_header_navlink_app/pre

Rebased

PHPBB3-12346
[ticket/12346] Change tabs to spaces

PHPBB3-12346
Merge pull request #2233 from PayBas/ticket/12346

[ticket/12346] Add Template Event overall_header_navlink_append/prepend

* PayBas/ticket/12346:
  [ticket/12346] Change tabs to spaces
  [ticket/12346] Add Template Event overall_header_navlink_app/pre