allow to change the event in the triggered event handlers

Currently if you register an event handler:
```
links.events.addListener(mytimeline, 'add', modifyEventText);
```
Changes to the event inside the modifyEventText method are not displayed because in timeline.js  the item is retrieved (on line 2938) before calling the handler (on line 2952) and is not updated after the handler and before calling updateData.

This simple PR fixes that :)