Update mailer.php

+ changed "$this->params->get('receivers')" to "$this->_receivers" to allow admin mails to system mail receivers only (empty "receivers" field)
+ completely reworked onEventEdited() and onVenueEdited() to allow new events/venues
  (2nd param "edited" is "isNew" by sender (inverse), "$edited" was used multiple times with different meaning - now named $isNew or $modified)
+ _mailer() prepared to use "BCC" instead of "To" - requires new $data->useBCC flag to implement (incl. backend setting)