New widget: About Dialog
I'm starting to think that patching the GTK About Dialog was a mistake, and we should just go for it and make an adaptive dialog in Libhandy. About dialogs are a very explicitly GNOME-y pattern that we'll want to keep iterating on, so for GTK 4 it should probably go into the HIG library rather than the core toolkit.
I think prototyping it in Libhandy for GTK 3 makes sense, especially if it's a drop-in replacement for apps in terms of API. IMO some apps having different About dialogs for a while is not a real problem, there are much more visible differences between adaptive and non-adaptive apps :)