This commit revamps the 'send & print' wizard.

Previously when opening the wizard, 2 PDF were rendered, 1 was added to the move.
When clicking "Send & Print" in the wizard, another PDF was added to the move,
and the other attachments were also duplicated on the move.

Now:

We avoid creating a PDF on the move when opening the wizard, using a context key.

In addition, when opening the wizard, the move's attachments
are *linked* to the wizard rather than recreated, s.t. when closing the
wizard, the attachments (except the PDF) are not duplicated on the move.

When using the context key, a weird bug occured (template was
not loaded directly, empty wizard, as reported in
3ae25d6).
Changing account.invoice.send from delegation inheritance to
classical inheritance solved it.

It also prevents us from generating 2 times a PDF when opening
the wizard (it was the case previously).

If we choose the option 'Print' in the wizard, we will still have a duplicated
PDF if the filename in the email template is different from:
{{ (object.name or '').replace('/','_') }}{{ object.state == 'draft' and '_draft' or '' }}
Indeed, the ir.actions.reports triggers the _render_qweb_pdf, which by default
regenerate a new PDF and creates a new attachment on the move if it doesn't already
exists (see retrieve_attachment in ir.actions.report).

task-???

Odoo PR: https://github.com/odoo/odoo/pull/96674