The StyleResolver can find the specified CSS values for the parent
element via the DOM. Forcing everyone to locate specified values for
their parent was completely unnecessary.
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.
LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
