[ticket/12943] Add core.phpbb_generate_debug_output core event

Add core.phpbb_generate_debug_output core event to allow extensions
outputting additional debug info.

PHPBB3-12943
Merge pull request #2846 from rxu/ticket/12943

[ticket/12943] Add core.phpbb_generate_debug_output core event

* rxu/ticket/12943:
  [ticket/12943] Add core.phpbb_generate_debug_output core event