Selectmenu: Introduce _renderButtonItem method

Fixes #10142
Very much unnecessary. The extension point is for modifying the item, not the parent. There should be no assumption that you can walk the tree like this. 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.