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.cpp

@@ -20,8 +20,4 @@ BreakNode::~BreakNode()
 {
 }
 
-void BreakNode::paint(PaintContext&, Painting::PaintPhase)
-{
-}
-
 }