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.
