LibWeb: Rename LayoutNode classes and move them into Layout namespace

Bring the names of various boxes closer to spec language. This should
hopefully make things easier to understand and hack on. :^)

Some notable changes:

- LayoutNode -> Layout::Node
- LayoutBox -> Layout::Box
- LayoutBlock -> Layout::BlockBox
- LayoutReplaced -> Layout::ReplacedBox
- LayoutDocument -> Layout::InitialContainingBlockBox
- LayoutText -> Layout::TextNode
- LayoutInline -> Layout::InlineNode

Note that this is not strictly a "box tree" as we also hang inline/text
nodes in the same tree, and they don't generate boxes. (Instead, they
contribute line box fragments to their containing block!)

Libraries/LibWeb/HTML/HTMLElement.cpp

@@ -27,7 +27,7 @@
 #include <AK/StringBuilder.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/HTMLElement.h>
-#include <LibWeb/Layout/LayoutText.h>
+#include <LibWeb/Layout/TextNode.h>
 
 namespace Web::HTML {
 
@@ -117,10 +117,10 @@ String HTMLElement::inner_text()
     if (!layout_node())
         return text_content();
 
-    Function<void(const LayoutNode&)> recurse = [&](auto& node) {
+    Function<void(const Layout::Node&)> recurse = [&](auto& node) {
         for (auto* child = node.first_child(); child; child = child->next_sibling()) {
             if (child->is_text())
-                builder.append(downcast<LayoutText>(*child).text_for_rendering());
+                builder.append(downcast<Layout::TextNode>(*child).text_for_rendering());
             if (child->is_break())
                 builder.append('\n');
             recurse(*child);