[IMP] web: list multi-edit: discard, cancel then save

In a list view that allows the multi edition, select several
records, edit a field, and 'click' on Discard. In the confirm
dialog, click on 'Cancel'. The row is still in edition and the
changes are still there. However, before this commit, clicking
on 'Save' did nothing.

The reason is that the 'Save' button is fake. Typically, clicking
on it triggers a 'field_changed' event, thus saving the selected
records. In this above scenario, the changes have already been
notified, so nothing happens when the user clicks on 'Save'.

This commit fixes the issue by storing the last 'field_changed'
event, and re-triggering it when necessary. This isn't the cleaner
way to handle that scenario, but it is the simpler, so we go for
it in stable. In master, it would be nice to rework the multi
edition feature with this scenario in mind.

task-2376233

closes odoo/odoo#62652

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>