In addition to the checkbox, the tableRowGroup component in our table also needs
to be aware of what rows are selected. We can accomplish this by binding it's
selected attribute to the selected property of our managed bean.
Additionally, the column containing the checkboxes needs to be aware of the
component id of the component used to select a row (the checkbox in our example).
For this we need to set the value of it's selectId attribute to match the value of the
id attribute of the checkbox.