notebook: Implement rtl flipping for CSS nodes

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.

testsuite/css/nodes/notebook.bottom.rtl.nodes

@@ -0,0 +1,18 @@
+[window.background:dir(rtl)]
+  decoration:dir(ltr)
+  notebook.frame:dir(rtl)
+    stack:dir(ltr)
+      button#page1.text-button:dir(rtl)
+        label:dir(ltr)
+      button#page2.text-button:dir(rtl)
+        label:dir(ltr)
+    header.bottom:dir(ltr)
+      button#end.text-button:dir(rtl)
+        label:dir(ltr)
+      tabs:dir(ltr)
+        tab:dir(ltr)
+          label#tab2:dir(ltr)
+        tab:active:dir(ltr)
+          label#tab1:dir(ltr)
+      button#start.text-button:dir(rtl)
+        label:dir(ltr)