[ticket/12524] Add search.php core event to modify search results rowset

Currently it is possible to change search results within the search row scope,
but it should be possible to modify/unset every individual row
within the rowset.

PHPBB3-12524
Merge pull request #2437 from rxu/ticket/12524

[ticket/12524] Add search.php core event to modify search results rowset

* rxu/ticket/12524:
  [ticket/12524] Add search.php core event to modify search results rowset
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12524] Add search.php core event to modify search results rowset