style: Distinguish between the tree structures used for traversal and selector matching.

This patch renames TNode::parent_element to traversal_parent, since it returns
the parent from the perspective of traversal (which in Gecko uses the
flattened tree).  It also renames TNode::children to traversal_children
for the saem reason.

We keep parent_element and children functions on TNode to use for selector
matching, which must be done on the real DOM tree structure.

components/style/matching.rs

@@ -230,7 +230,7 @@ trait PrivateMatchMethods: TElement {
     fn layout_parent(&self) -> Self {
         let mut current = self.clone();
         loop {
-            current = match current.parent_element() {
+            current = match current.traversal_parent() {
                 Some(el) => el,
                 None => return current,
             };