Add selection listener to Polymer example.
Add selection listener to Polymer example.

Peppe/vaadin-components-examples,tehapo/components-examples,manolo/components-examples,shahrzadmn/components-examples,tehapo/components-examples,shahrzadmn/components-examples,manolo/components-examples,Peppe/vaadin-components-examples,shahrzadmn/components-examples,Peppe/vaadin-components-examples,manolo/components-examples,tehapo/components-examples