This is probably my final big merge request for GNOME 42. This is a major cleanup on how panel headerbars are handled. It achieves the following:
- Move the right headerbar (the panel's headerbar) into panels themselves
- Rework the Display panel to have one headerbar per leaflet page
- Cleanup some widgetry
Some of the commits are big, but I think everything in this series makes sense. I'm quite happy with how it turned out