[FIX] stock: quant list editable bottom

Before this commit, the editable quant list was editable by top, which
is not really pratical when the user creates (with the keyboard)
multiple quants tracked by SN because the focus always go back on the
top of the list. Now, this list is editable by bottom.

closes odoo/odoo#38704

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>