BHV-5139: moon.CheckboxItem: Add API for changing checkmark icon

### Issue:
Add API to moon.CheckboxItem to change checkmark icon to other asset/icon, to support things like "locked"/"unlocked", etc.
### Fix:
Change moon.Checkbox to use a moon.Icon component, instead of styling the checkmark directly. This required some css adjustments. Then add public properties, "locked", "icon", and "src" to moon.CheckboxItem, and propagate property changes to the moon.Checkbox component.

Enyo-DCO-1.1-Signed-off-by: Krishna Rangarajan <krishna.rangarajan@lge.com>