[REF] web: force dropdown menu icon through getter

Before this ref, the dropdown menu got his icon directly from his props.
This means that any inheriting class had either to define its icon with
the 'default_props' key or to have its parent giving it in the props.

Now, the icon is retrieved from the props via a getter (same as title
and items) and can be overridden to force a fixed icon on the class.