[FIX] web: too small tap area for optional columns dropdown on mobile

Before this commit, users encountered some difficulties to tap/click
on the "optional columns" dropdown in list views.
This issue only occured on Chrome: rather annoying to use the mobile app.

"width: 10px" was used in inline list view but didn't have any sense
in the other case. Worse still, Chrome miscalculated the size of the
clickable area by a smaller one.
This is due to the fact that we add some padding that exceed 10px.
See:
https://github.com/odoo/enterprise/commit/2f1320557ec67a741201d73a23a9782ec8f9b3bb#



opw-2071605

closes odoo/odoo#42183

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>