Add a placeholder to beautify the UX of mail preview wizard. Field is currently a bit hard to sport with the new UI. closes odoo/odoo#105751 X-original-commit: fd5a5bc38bad09e3e8e99ff22e64a5dda60e32aa Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>