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/Forward.h

@@ -26,10 +26,6 @@
 
 #pragma once
 
-namespace Web {
-enum class LayoutMode;
-}
-
 namespace Web::CSS {
 class Selector;
 class StyleProperties;
@@ -151,9 +147,18 @@ class SVGSVGElement;
 }
 
 namespace Web::Layout {
+enum class LayoutMode;
+class BlockBox;
 class BlockFormattingContext;
+class Box;
+class ButtonBox;
+class CheckBox;
+class InitialContainingBlockBox;
 class FormattingContext;
 class InlineFormattingContext;
+class Node;
+class NodeWithStyle;
+class ReplacedBox;
 }
 
 namespace Web {
@@ -161,14 +166,6 @@ class EventHandler;
 class Frame;
 class FrameLoader;
 class InProcessWebView;
-class LayoutBlock;
-class LayoutBox;
-class LayoutButton;
-class LayoutCheckBox;
-class LayoutDocument;
-class LayoutNode;
-class LayoutNodeWithStyle;
-class LayoutReplaced;
 class LineBox;
 class LineBoxFragment;
 class LoadRequest;