feat #1231 Selected option highlight in selectbox widget

On click of selectbox widget, the selected option does not get highlighted in the dropdown. 
Example: http://instant.ariatemplates.com/anonymous/0dc229a5c8c664f9e3a3/f305c3
This commit makes the selected option highlighted in the dropdown.
There is no need to provide `sgs` as a parameter here. Since it is equal to `this._options`, it can be accessed to directly within method `_getSelectedIndex`