[ticket/12336] Add functions_module.php core event

Add functions_module.php core event to allow adjusting parameters
for custom ACP, MCP, UCP modules (location: function list_modules()).

PHPBB3-12336
[ticket/12386] Add DEBUG_EXTRA again and use it for container creation

We are currently creating a new compiled container on every page load when
having DEBUG enabled. However, one might only have that enabled to be
presented with errors or for getting the page load stats.
This change will add the DEBUG_EXTRA constant again. It will be used for
choosing whether the compiled container should be created on every page load -
when defined as true - or just once after the cache is cleared.

PHPBB3-12386
[ticket/12386] Rename DEBUG_EXTRA to DEBUG_CONTAINER for more descriptive name

PHPBB3-12386
Merge remote-tracking branch 'marc/ticket/12386' into develop-ascraeus

* marc/ticket/12386:
  [ticket/12386] Rename DEBUG_EXTRA to DEBUG_CONTAINER for more descriptive name
  [ticket/12386] Add DEBUG_EXTRA again and use it for container creation
Merge remote-tracking branch 'rxu/ticket/12336' into develop-ascraeus

* rxu/ticket/12336:
  [ticket/12336] Add functions_module.php core event