LibWeb: Process style sheets in document order

Until now we would simply apply stylesheets in the order they finished
loading. This patch adds a StyleSheetList object that hangs off of each
Document and contains all the style sheets in document order.

There's still a lot of work to do for a proper cascade, but at least
this makes us consistently wrong every time. :^)

Libraries/LibWeb/DOM/Document.cpp

@@ -63,6 +63,7 @@ namespace Web {
 Document::Document(const URL& url)
     : ParentNode(*this, NodeType::DOCUMENT_NODE)
     , m_style_resolver(make<StyleResolver>(*this))
+    , m_style_sheets(CSS::StyleSheetList::create(*this))
     , m_url(url)
     , m_window(Window::create_with_document(*this))
 {