Sticky rows

[BierDav]

I want to display groups in my table and make specific rows sticky, the problem is that isHeader is not enough for this usecase. Because currently there is no way to know if the row is currently in viewport or not and on the other hand there is no way to set an offset for the header it self. If we had this information and property, we could implement sticky headers ourself

[sproctor]

I would certain accept a PR to do this. I'm not sure if having a sticky flag makes more sense.