Add event manager as soft dependency to translator

This PR allows users to attach to two new events in the translator:

- loadMessages.no-messages-loaded which will always fire when messages for a specific locale/text-domain combination could not be loaded
- getTranslatedMessage.missing-translation which will always fire when a message in a specific locale/text-domain could not be found

By default, events are disabled, and the translator will not try to create an event manager instance. This makes it a rather nice soft dependency, without having it as requirement.