SelectBox adjustmets
SelectBox adjustmets

* doc of props
* selected prop is marked as optional
* it renders empty dropdown correctly `items`, previously it:
  * displays text '&nbsp'
  * showed one empty option
  * was firing onChange event eventhough nothing souldn't be selectable

oVirt/ovirt-web-ui,mareklibra/userportal,oVirt/ovirt-web-ui,oVirt/ovirt-web-ui,mareklibra/userportal,mareklibra/userportal,mareklibra/userportal