LibWeb: Add method for listing all style sheets on a page

This will be used by the inspector, for showing style sheet contents.

Identifying a specific style sheet is a bit tricky. Depending on where
it came from, a style sheet may have a URL, it might be associated with
a DOM element, both, or neither. This varied information is wrapped in
a new StyleSheetIdentifier struct.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -102,6 +102,7 @@ set(SOURCES
     CSS/StyleProperties.cpp
     CSS/StyleProperty.cpp
     CSS/StyleSheet.cpp
+    CSS/StyleSheetIdentifier.cpp
     CSS/StyleSheetList.cpp
     CSS/StyleValues/AngleStyleValue.cpp
     CSS/StyleValues/BackgroundRepeatStyleValue.cpp