[list-box] Add disabled property

[yuriy-fix]

One of the participants suggested to introduce disabled attribute on vaadin-list-box.

[web-padawan]

The disabled attribute is supposed to be used on form controls, especially by whatwg spec. I'm not sure if the list-box should be used as a standalone control.

[jouni]

<vaadin-list-box> has been meant as a more versatile replacement for native <select size="4">, which does have disabled. Though, list box currently doesn’t have a value, so it’s a bit weird in that regard.

Maybe there’s some misalignment about the role of this component?