Remove <field name="property_account_position_id"> from warning banner
because other modules inherit the partner form view and put fields after
property_account_position_id (example : OCA module intrastat_product)
and we don't want those fields to be displayed in the warning banner!
A yellow warning banner is now displayed on the partner form view when
the fiscal position requires a VAT number and the VAT number is missing.
This banner replaces the warning message triggered by the onchange.
Improve README of the module with v14 screenshots.