Revert "[FIX] stock_picking_wave: removed obsolete many2many widget from stock_picking_wave_view"

This reverts commit 436bbb0a.

The many2many widget is usefull, as it allows to select an existing picking instead of creating a new one when clicking add an item from the list

addons/stock_picking_wave/stock_picking_wave_view.xml

@@ -24,7 +24,7 @@
                             <field name="user_id"/>
                         </group>
                         <separator string="Pickings"/>
-                        <field name="picking_ids" domain="[('state', 'not in', ('done', 'cancel'))]">
+                        <field name="picking_ids" widget="many2many" domain="[('state', 'not in', ('done', 'cancel'))]">
                             <tree>
                                 <field name="name"/>
                                 <field name="backorder_id"/>