Selectmenu: Introduce _renderButtonItem method

Fixes #10142
> In fact, this makes me change my mind and say that this is not a good implementation. _renderButtonItem() should be generating the item, just like _renderItem() does. That item should then be appended to the button.

This way we would need to append the element on create and replace the element on each select. 

Keep in mind that _renderItems is called once on create or by refresh(). _renderButtonItem is called on create, refresh() and on each item select as we need to update button content.