DataGrid: add API that tells us if detail row is expanded or present

[stsrki]

          > Yes, I understand it is private for a reason. Question is: should I try to came up with a solution for exposing the `HasDetailRow` state? I guess that would go to `master` branch so the answers doesn't block merging of this pr.

I guess it might be worth it. It's good to know that information. Now that we are pursuing this. I think HasDetailRow is a weird name. So we can try having something more descriptive like DetailRowExpanded.

I will open new issue.

Originally posted by @stsrki in #5940 (comment)