[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
This should probably go in a link module (e.g. mail_tip)