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/HTMLLinkElement.h

@@ -50,6 +50,8 @@ private:
     virtual void resource_did_load() override;
 
     void load_stylesheet(const URL&);
+
+    RefPtr<StyleSheet> m_style_sheet;
 };
 
 template<>