Add extra class for group head when child items are revealed

[rytswd]

It would be great if I can adjust the style of the group module to have slightly different colour / border / etc. only when the group child items are revealed.

Hidden	Revealed

I had a quick look at the code but could not find a reference to [gtk_revealer_get_child_revealed](https://docs.gtk.org/gtk3/method.Revealer.get_child_revealed.html) -- so I'm assuming this is not something I can add extra styling for just yet, but hopefully not too difficult to support? (I hardly have any knowledge on C++, apologies if I'm talking nonsense!)