LibHTML: Refactor to go from DOM -> styled tree -> layout tree.

Frame::layout() drives everything now, it takes the DOM contained in the
frame and puts it through the tree transformations.

LibHTML/CSS/StyledNode.h

@@ -8,6 +8,12 @@
 
 class Node;
 
+enum class Display {
+    None,
+    Block,
+    Inline,
+};
+
 class StyledNode : public TreeNode<StyledNode> {
 public:
     static NonnullRefPtr<StyledNode> create(const Node& node)
@@ -44,6 +50,8 @@ public:
         m_property_values.set(name, move(value));
     }
 
+    Display display() const;
+
 protected:
     explicit StyledNode(const Node*);