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/CSS/Length.cpp

@@ -31,7 +31,7 @@
 
 namespace Web::CSS {
 
-float Length::relative_length_to_px(const LayoutNode& layout_node) const
+float Length::relative_length_to_px(const Layout::Node& layout_node) const
 {
     switch (m_type) {
     case Type::Ex: