[IMP] pos, l10n_fr_pos_cert: change price_manually_set behaviour

The price_manually_set variable was used to largely, we fix it
by adding a price_automatically_set variable. The price_manually_set
is now used when changing the price through the numpad, whereas
price_manually_set is used to indicate that the price is not
the price set in the database.

closes odoo/odoo#118004

Signed-off-by: Heinz Robin (rhe) <rhe@odoo.com>