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/BoxModelMetrics.cpp

@@ -26,9 +26,9 @@
 
 #include <LibWeb/Layout/BoxModelMetrics.h>
 
-namespace Web {
+namespace Web::Layout {
 
-PixelBox BoxModelMetrics::margin_box(const LayoutNode& layout_node) const
+PixelBox BoxModelMetrics::margin_box(const Node& layout_node) const
 {
     return {
         margin.top.to_px(layout_node) + border.top.to_px(layout_node) + padding.top.to_px(layout_node),
@@ -38,7 +38,7 @@ PixelBox BoxModelMetrics::margin_box(const LayoutNode& layout_node) const
     };
 }
 
-PixelBox BoxModelMetrics::padding_box(const LayoutNode& layout_node) const
+PixelBox BoxModelMetrics::padding_box(const Node& layout_node) const
 {
     return {
         padding.top.to_px(layout_node),
@@ -48,7 +48,7 @@ PixelBox BoxModelMetrics::padding_box(const LayoutNode& layout_node) const
     };
 }
 
-PixelBox BoxModelMetrics::border_box(const LayoutNode& layout_node) const
+PixelBox BoxModelMetrics::border_box(const Node& layout_node) const
 {
     return {
         border.top.to_px(layout_node) + padding.top.to_px(layout_node),