LibWeb: Add a simple StyleInvalidator class

This patch adds a simple, naive & inefficient class for document-wide
style invalidation, e.g. after element attribute updates. During
construction it collects a HashMap of a document's elements and their
matching rules, during destruction it does the same and then compares
the results; dirtying all elements that have a different number or order
of matching rules afterwards.

Much room for improvement, but it solves the problem of stale element
styling after attribute updates for now :^)

Fixes #4404.

Libraries/LibWeb/CMakeLists.txt

@@ -23,6 +23,7 @@ set(SOURCES
     CSS/Selector.cpp
     CSS/SelectorEngine.cpp
     CSS/StyleDeclaration.cpp
+    CSS/StyleInvalidator.cpp
     CSS/StyleProperties.cpp
     CSS/StyleResolver.cpp
     CSS/StyleRule.cpp