CAD — adding an attribute to the entity group

Our entity groups in Cad Services don't have any form of an
identifier or name that can be retrieved through the interface.
The only returned data is either a list of entities in the group or
the count. It makes it difficult to present this information to a
user.

We might consider adding an attribute to the entity group,
however that means it gets inherited to the layer.

readonly attribute string native_label;

A readonly attribute was added to the EntityGroup interface.