LibHTML: Move layout tree building to a LayoutTreeBuilder class

Building a whole layout tree shouldn't be a concern of Node, so this
patch moves it to a separate class.

Libraries/LibHTML/Layout/LayoutTreeBuilder.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <AK/RefPtr.h>
+
+class Node;
+class LayoutNode;
+
+class LayoutTreeBuilder {
+public:
+    LayoutTreeBuilder();
+
+    RefPtr<LayoutNode> build(Node&);
+};