[IMP] point_of_sale: visual improvement

Small visual changes that should improve the user experience.
The loading demo data popup has been moved to the backend.
Created an Alert Popup that is less aggressive than an Error Popup

closes odoo/odoo#83780

Task-id: 2485062
Related: odoo/enterprise#26104
Signed-off-by: Masereel Pierre <pim@odoo.com>