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/HTMLBlinkElement.cpp

@@ -28,7 +28,7 @@
 #include <LibWeb/CSS/StyleProperties.h>
 #include <LibWeb/CSS/StyleValue.h>
 #include <LibWeb/HTML/HTMLBlinkElement.h>
-#include <LibWeb/Layout/LayoutNode.h>
+#include <LibWeb/Layout/Node.h>
 
 namespace Web::HTML {