[FIX] web_tip: wait for chatter to fetch messages + various improvements

* Override message_fetch from mail.Thread and fire an event when the messages are loaded. This is necessary for tips on the chatter
* Override render_value form web.form.FieldStatus : Call reposition() when FieldStatus is done rendering.
* Tip.reposition() method
* Handle window resize event
should probably use `openerp.web.bus.on('resize', …)` handler instead, so we only hook a single event to onresize and (if necessary) we have a single place where we can rate-limit that event.