Editor: Remove UIListbox namespace from ListboxItem.

[puxiao]

I get an error when I use UIListbox .

Uncaught TypeError: UIListbox.ListboxItem is not a constructor
at UIListbox.render (ui.js:1213:30)

https://github.com/mrdoob/three.js/blob/dev/editor/js/libs/ui.js

class UIListbox extends UIDiv { ... }

class ListboxItem extends UIDiv { ... }

You can see that: UIListbox and ListboxItem, they are independent of each other.

So UIListbox.ListboxItem is wrong.

- const listitem = new UIListbox.ListboxItem(this);
+ const listitem = new ListboxItem(this);