Hibernate mapping of Event object (rebased from dev_5_0)

This PR should leave the resulting DB schema unchanged. To be sure of this, have a look at the definition of the `event` table as created from `bin/omero db script` both without and with this PR. Also make sure that, even with this PR, after the DB is created, the server starts and works fine.

The point of this PR is to remove inaccuracy in the Hibernate mapping of the Event object.

--rebased-from #2623