Need to support placeholder text for SelectBox

[mmuzamil]

We should add a placeholder in the defaults for SelectBox. Whenever there is no entry selected in the SelectBox then this placeholder text should be displayed .

[aaronj1335]

👍

we should also support setting a placeholder through the placeholder attribute of the DOM element, which makes this settable in cases where the SelectBox is automatically instantiated, such as .widgetizeDescendents().