Add event to acp_main.php to notice admin

Like the notice on the main acp page, extensions should be able to generate their own notice for the admin. There is already a html event acp_main_notice_after so it should be able to do their own notice.

http://area51.phpbb.com/phpBB/viewtopic.php?f=111&t=45987
Use `$phpbb_dispatcher->dispatch('core.acp_main_notice');` if there are not parameters (without the `$vars = ...` and without the `extract()`)