ENYO-258 Add active property to checkboxItem

and bound with active property of checkbox.

Initially this.selected is null

Enyo-DCO-1.1-Signed-off-by: David Um david.um@lge.com
ENYO-258 Remove duplicated work

Binding active property did same job with initializedActiveItem.
if we leave this method, active item is pushed to selected twice.
So we don't need this method anymore.

Enyo-DCO-1.1-Signed-off-by: David Um david.um@lge.com
Merge pull request #1740 from enyojs/ENYO-258-davidum

enyo-258  ExpandablePicker: Selecting item with enter key does not change active status of checkboxItem

Reviewed-by: Roy Sutton (roy.sutton@lge.com)
Integrated-By: Ryan Duffy (ryan.duffy@lge.com)