[FIX] web: list: optional field dropdown expands toward the list

Be in RTL mode, on a list view
Click on the button to set up optional fields

Before this commit, the dropdown expanded to the right, and was partly invisible

After this commit, we dynamically set to which direction the dropdown should expand
depending on RTL/LTR
Consequently the dropdown is fully visible in either mode, and expands to the list
rather than away from it

closes #39402

closes odoo/odoo#39558

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>