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/Forward.h

@@ -485,6 +485,7 @@ namespace Web::Painting {
 class ButtonPaintable;
 class CheckBoxPaintable;
 class LabelablePaintable;
+class MediaPaintable;
 class Paintable;
 class PaintableBox;
 class PaintableWithLines;