[ticket/12513] Add simple_header head event with support for CSS assets calls

PHPBB3-12513
[ticket/12513] Add asset include support to simple header/footer in ACP

PHPBB3-12513
Merge pull request #2424 from VSEphpbb/ticket/12513

[ticket/12513] Add simple_header head event with support for asset calls

* VSEphpbb/ticket/12513:
  [ticket/12513] Add asset include support to simple header/footer in ACP
  [ticket/12513] Add simple_header head event with support for CSS assets calls