How to edit Nullable field with the ASPxGridView and the ASPxImage in the EditItemTemplate

The example demonstrates how to edit the Boolean field that allows writing "null" values (Nullable<Boolean>, Boolean?).
As the standard checkbox doesn't allow having three states (checked, unchecked and gray), the scenario can be implemented with the ASPxImage and three images. The images are taken from the default CSS sprite used by the ASPxEditors suite.