And incidentally make the CSS node docs correct again.
We were not propagating direction changes in some places, evidently. Now we do.
This is fallout from changing the tab states.
Use gtk_box_gadget_reverse_children and gtk_css_node_reverse_children to flip the children of the header_gadget and the tabs_gadget when appropriate. Add new CSS node tests to verify that the node order is updated as expected in all cases.
