[IMP] Event :Add action in more menu Allow to change the stage of severa...

...l Attendees at a time

Task: https://www.odoo.com/web#id=10877&view_type=form&model=project.task&menu_id=3942&action=327
PAD: https://pad.odoo.com/p/r.VA3tLJSeEPBPfA48