Don't link group heading format to first row in group

[dbaum]

@David_Joycenoted what seems like a bug--or, at least, suboptimal behavior in
Stop group headings using specific row formatting. @Aleksi provided a straightforward workaround. However, that's inefficient and it remains counterintuitive that a group heading is governed by the format rule condition of the first row in the group. Maybe format rules could have an option to apply to rows, group headings, or both (independently--not propagated from the first row to the group heading).