LibWeb: Extract media element timeline painting to a base class

This moves the painting of the media timeout out of VideoPaintable into
a base MediaPaintable. This is to allow re-using the same timeline logic
and controls for audio elements.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -454,6 +454,7 @@ set(SOURCES
     Painting/InlinePaintable.cpp
     Painting/LabelablePaintable.cpp
     Painting/MarkerPaintable.cpp
+    Painting/MediaPaintable.cpp
     Painting/NestedBrowsingContextPaintable.cpp
     Painting/PaintContext.cpp
     Painting/Paintable.cpp