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/HTMLCanvasElement.cpp

@@ -30,7 +30,7 @@
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/CanvasRenderingContext2D.h>
 #include <LibWeb/HTML/HTMLCanvasElement.h>
-#include <LibWeb/Layout/LayoutCanvas.h>
+#include <LibWeb/Layout/CanvasBox.h>
 
 namespace Web::HTML {
 
@@ -55,12 +55,12 @@ unsigned HTMLCanvasElement::height() const
     return attribute(HTML::AttributeNames::height).to_uint().value_or(150);
 }
 
-RefPtr<LayoutNode> HTMLCanvasElement::create_layout_node(const CSS::StyleProperties* parent_style)
+RefPtr<Layout::Node> HTMLCanvasElement::create_layout_node(const CSS::StyleProperties* parent_style)
 {
     auto style = document().style_resolver().resolve_style(*this, parent_style);
     if (style->display() == CSS::Display::None)
         return nullptr;
-    return adopt(*new LayoutCanvas(document(), *this, move(style)));
+    return adopt(*new Layout::CanvasBox(document(), *this, move(style)));
 }
 
 CanvasRenderingContext2D* HTMLCanvasElement::get_context(String type)