[REF] stock: immediate and backorder wizard multi

followup of rev [0]

If these wizards are called on multiple pickings, display the list of
the pickings that could be impacted and allow to select which one should
be impacted.

We also adapt the sanity checks at the start of `button_validte` in
order to specify the concerned pickings if needed. We do not enable the
multi behavior for batch at the moment, so it's only enabled for the
validate multi in the list view.

[0] 6ab4b0d4

task-2069646

closes odoo/odoo#41497

X-original-commit: ff276c6484b138982f185ec9c832f2dbf25baaef
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>