[ticket/12691] Add core.delete_pm to funtion delete_pm

Add core.delete_pm to funtion delete_pm.
Event will return:
int $user_id - ID of the user requested the message delete
array $msg_ids - array of all messages to be deleted
int $folder_id - ID of the user folder where the messages are stored
Justification:
Allow extensions to capture this event
and act as intended by ext author

PHPBB3-12691
The event name should indicate whether it is triggered before or after the action (in this case before)