[IMP] event: refactored event form view. Usability reviews
showed that the form view was complicated and difficult to use. Now
that stat button and edition options in the front-end exist, it is
possible to remove some of the numerous tabs as well as some options
from the form view.

Coming: further cleaning of the model, behavior and probably the various
event-related views.
Merge branch 'master' of https://github.com/odoo/odoo into master-linux64