This patch moves the Selector object model closer to the specification
objects in Selectors Level 4.
A "Selector" in LibHTML is now a { Vector<ComplexSelector> }, which is
a { Relation, CompoundSelector }. A CompoundSelector is really just
a Vector<SimpleSelector>, and SimpleSelector is "Component" renamed.
This makes a lot more selectors actually match on the Ubuntu Apache2
default homepage. :^)