LibWeb: Let Paintable perform the painting

This patch adds a bunch of Paintable subclasses, each corresponding to
the Layout::Node subclasses that had a paint() override. All painting
logic is moved from layout nodes into their corresponding paintables.

Paintables are now created by asking a Layout::Box to produce one:

    static NonnullOwnPtr<Paintable> Layout::Box::create_paintable()

Note that inline nodes still have their painting logic. Since they
are not boxes, and all paintables have a corresponding box, we'll need
to come up with some other solution for them.

Userland/Libraries/LibWeb/Layout/BreakNode.h

@@ -20,7 +20,6 @@ public:
 
 private:
     virtual bool is_break_node() const final { return true; }
-    virtual void paint(PaintContext&, Painting::PaintPhase) override;
 };
 
 template<>