Fix missing entry properties passed to 'backend_publish' event hook (Fixes #206)

Some plugins make use of additional entry properties (title, body, extended, author) after being called by the 'backend_publish' hook, e.g.

 * serendipity_event_forum
 * serendipity_event_freetag
 * serendipity_event_ljupdate
 * serendipity_event_trackback
 * serendipity_event_versioning
 * serendipity_event_entryproperties
 * serendipity_event_mailer
 * serendipity_event_twitter

 Thus we need to set these properties in the serendipity_updertEntry() method to make sure the plugins get the required data. Of course, we could fetch the entry either in overview.inc.php or again in each plugin, but that would mean an additional query to the DB.