1. Rename the thing
2. Turn it from a signal to a vfunc
3. Pass the GtkCssStyleChange to it
We don't export any public API about the GtkCssStyleChange yet, it's
just a boring opaque struct.
Instead of having old and new style, now have a GtkCssStyleChange opaque
object that will compute the changes you are interested in for you.
This simplifies change signal handlers quite a bit and avoids lots of
repeated computation in every signal handler.