[IMP] mail: Inbox: usability improvements: allow to search on the document type (model) using a new model_id field on mail.message; some usability fixes on the chatter widget

I think it is not the best way to handle it. I would use model_id as a normal many2one field, and model set as a related field (maybe function, so that setting model also sets model_id). Then searching on it will be straightforward.