Lib: Fix event interface

`target` and friends are set to `null` when you create them manually
There is no interface to create event with js_of_ocaml. However, eliom does the following : 
1- create event : https://github.com/ocsigen/eliom/blob/master/src/client/private/eliommod_dom.ml#L221.
2- manually dispatch the event : https://github.com/ocsigen/eliom/blob/master/src/client/eliom_client.ml#L406

this result in having event with `target` set to `null`

 - eliom should not dispatch event manually .. @balat 
 - fix event class type (this PR) ? @vouillon