[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.