Allowed checkbox item to be set to `blank` or anything else in a `DataGridList`.

Enyo-DCO-1.1-Signed-off-by: Blake Stephens <blake.stephens@lge.com>
Merge pull request #1678 from enyojs/BHV-15435-blakestephens

BHV-15435: Allowed custom checkmarks on `CheckboxItem` to be set when in a `DataGridList`

Reviewed-By: Krishna Rangarajan (krishna.rangarajan@lge.com)
Integrated-By: Ryan Duffy (ryan.duffy@lge.com)