- GtkCssWidgetNode
for style contexts owned by a widget
- GtkCssPathNode
for style contexts using a GtkWidgetPath
- GtkCssTransientNode
for nodes created with gtk_style_context_save()/restore()
The functionality of it is supposed to grow, so better put it in a
custom file early.
This is just a naive split so far, the next patches will split things
further.