[FIX] mail: keep chat window hidden on receiving new message

Before this commit, when receiving a new message from a hidden chat
window, it was making it visible. Because of this, if we had focus
on any other window, the focus was lost. Even more annoying, if we
had focus on the left-most visible chat window, it moved to hidden
menu, so that new chat window takes its place.

This commit fixes the issue by keeping new chat windows hidden
on receiving new messages, in case there is not enough available
space.

Task-ID 2026101

closes odoo/odoo#34291

Signed-off-by: Martin Geubelle (mge) <mge@openerp.com>