[FIX] web: scroll handling for many2one selection

If the many2one selection height was too big (bigger than the browser page), it wasn't possible to see all options, because the body is set as overflow: hidden;

Moreover, if you opened a many2one selection and then scrolled the page, the selection moved with the scrolling, while it should be sticked to its input field
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32