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/Layout/LayoutTreeBuilder.h

@@ -29,13 +29,13 @@
 #include <AK/RefPtr.h>
 #include <LibWeb/Forward.h>
 
-namespace Web {
+namespace Web::Layout {
 
 class LayoutTreeBuilder {
 public:
     LayoutTreeBuilder();
 
-    RefPtr<LayoutNode> build(DOM::Node&);
+    RefPtr<Node> build(DOM::Node&);
 };
 
 }