[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
The outer IIFE is not necessary here, just define `openerp.web_tip` at the top-level of the file.