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/CMakeLists.txt

@@ -24,6 +24,7 @@ set(SOURCES
     CSS/StyleResolver.cpp
     CSS/StyleRule.cpp
     CSS/StyleSheet.cpp
+    CSS/StyleSheetList.cpp
     CSS/StyleValue.cpp
     DOM/AttributeNames.cpp
     DOM/CanvasRenderingContext2D.cpp