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.
This tests that horizontal boxes flip their child nodes
according to text direction to maintain the left-to-right
ordering of child nodes for both text directions.
CSS nodes have a linear sibling relationship; this is supposed
to correspond to left-to-right placement in horizontal arrangements.
This commit explicitly sets the text direction to rtl if the
filename ends in .rtl.ui, so we can test differences in node
tree layout between text directions.
