[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#42210

X-original-commit: 28b6e9c99c77230dc19808718cd6f6f8aec0db5d
Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/web/static/src/scss/list_view.scss

@@ -300,7 +300,7 @@
 
     .o_optional_columns_dropdown_toggle {
         cursor: pointer;
-        width: 10px;
+        padding: 0 5px;
         text-align: center;
         line-height: 30px;
         z-index: 1; // must be over the resize handle