[IMP] stock: onboarding (display improvements)
[REF] stock: product quantity stat button

Clean version of stat button of product quantity. Will also work in
creation mode and avoid unuseful computed string field

(fix parts of commit 52c3b52bf09fea7e0920ebcec3f0e8acb066b2b7)
[REM] product: leftovers of unused field "mes_type"

Also removed in data in point_of_sale, project_timesheet and sale_mrp.
[FIX] stock: correct label for picking_id
[REF] stock: replace <div> in view by nolabel

This patch does not change the look of the view; it is only intended to
use the standard feature "nolabel" instead of hacking the HTML with a
<div> tag.
[REV] stock: partial revert of 52c3b52bf09fea7e0920ebcec3f0e8acb066b2b7

The <div> were introduced to correct alignement issues in this form view. Actually, they are generic and due to inherited invisible group, so CSS is sometimes incorrect when you hide groups. Should be fixed in form view implementation.
[IMP] allow many2many to specify formview (web client)

One2Many fields can specify sub tree and sub form view, to be used when
displaying a list view and when adding/editing an element.  Many2many
can display a sub tree view, but ignore sub formviews.  This can be
fixed by specifying an alternative_form_view to the popup form widget.
website_event_sale: validation of seats availability in cart.
Attendees are created and deleted on changing the quantity from the cart.

Also fixed check_auto_confirmation, now in multi mode to avoid issues
due to the specific behavior of api.one.
[FIX] Fixed the traceback occured while clicking on the CONFIRM SALE from quotations where self is used instead of sale_order and fixed the issue of warning which is not shown when no seats available in the cart. And added unlink method for cancelling all the attendees which didn't have any effect on them if the so_line related to them is direct removed(10 to directly 0 in cart.)