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.
This is kind of a hack the way it's implemented, but it's necessary
for performance to ignore transient nodes as they get created all the
time (via gtk_style_context_save()) and invalidate the whole treeview.
And that causes resizes and redrawing of the treeview and performance of
the inspector would go down the drain now that we display a larger part
of the node tree.
