Before, the small "caret" to the right of the dropdown was only
displayed if the current Dropdown was a child of another Dropdown.
It is sometimes necessary to display this caret in a parent
`Dropdown` in order to make the user understand that there is a
dropdown if he clicks on the text.
Now, it is possible thanks to a props to display manually this carret
if desired.
Part-of: odoo/odoo#99321
Dardenne Florent (dafl)
bd8afccf16