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/HTMLBRElement.cpp

@@ -25,7 +25,7 @@
  */
 
 #include <LibWeb/HTML/HTMLBRElement.h>
-#include <LibWeb/Layout/LayoutBreak.h>
+#include <LibWeb/Layout/BreakNode.h>
 
 namespace Web::HTML {
 
@@ -38,9 +38,9 @@ HTMLBRElement::~HTMLBRElement()
 {
 }
 
-RefPtr<LayoutNode> HTMLBRElement::create_layout_node(const CSS::StyleProperties*)
+RefPtr<Layout::Node> HTMLBRElement::create_layout_node(const CSS::StyleProperties*)
 {
-    return adopt(*new LayoutBreak(document(), *this));
+    return adopt(*new Layout::BreakNode(document(), *this));
 }
 
 }