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/CSS/StyleSheet.h

@@ -41,6 +41,7 @@ public:
     ~StyleSheet();
 
     const NonnullRefPtrVector<StyleRule>& rules() const { return m_rules; }
+    NonnullRefPtrVector<StyleRule>& rules() { return m_rules; }
 
 private:
     explicit StyleSheet(NonnullRefPtrVector<StyleRule>&&);