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!)

Applications/Browser/InspectorWidget.cpp

@@ -68,7 +68,7 @@ InspectorWidget::InspectorWidget()
 
     m_layout_tree_view = top_tab_widget.add_tab<GUI::TreeView>("Layout");
     m_layout_tree_view->on_selection = [this](auto& index) {
-        auto* node = static_cast<Web::LayoutNode*>(index.internal_data());
+        auto* node = static_cast<Web::Layout::Node*>(index.internal_data());
         set_inspected_node(node->dom_node());
     };