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/CMakeLists.txt

@@ -284,8 +284,20 @@ set(SOURCES
     Page/Page.cpp
     Painting/BackgroundPainting.cpp
     Painting/BorderPainting.cpp
-    Painting/Paintable.cpp
+    Painting/ButtonPaintable.cpp
+    Painting/CanvasPaintable.cpp
+    Painting/CheckBoxPaintable.cpp
+    Painting/ImagePaintable.cpp
+    Painting/MarkerPaintable.cpp
+    Painting/NestedBrowsingContextPaintable.cpp
     Painting/PaintContext.cpp
+    Painting/Paintable.cpp
+    Painting/ProgressPaintable.cpp
+    Painting/RadioButtonPaintable.cpp
+    Painting/SVGGeometryPaintable.cpp
+    Painting/SVGGraphicsPaintable.cpp
+    Painting/SVGPaintable.cpp
+    Painting/SVGSVGPaintable.cpp
     Painting/ShadowPainting.cpp
     Painting/StackingContext.cpp
     RequestIdleCallback/IdleDeadline.cpp