enyo-258  ExpandablePicker: Selecting item with enter key does not change active status of checkboxItem

Issue
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Selecting item with 5-way key and magic remocon makes different result.
In former case, selected item's `active` status does not changed

Cause
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`active` property is only in `moon.Chekcbox` not in `moon.CheckboxItem`

Fix
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Add `active` property to moon.CheckboxItem and bind it to active property of moon.Checkbox

Enyo-DCO-1.1-Signed-off-by: David Um david.um@lge.com