Confirm before sending notification email, fix #144

Popup a confirm modal (same as cozy-file) when a event with attendee is saved and had changed.

To do that, 
* add a dirty flag on event instances,
* dirty flag is update by change event, and by hand in detail view,
* popup the modal from model/event::sync method,
* add flag _sendMails_ in data sended to the server, to let him do the right things.