The idea here is to let us decide ahead of time what components to paint
depending on the size available. We currently paint each component left-
to-right, until we run out of room. This implicitly gives priority to
the left-most components.
We will soon paint volume controls on the right-side of the timeline.
Subjectively, they should have a higher priority than, say, the timeline
scrubbing bar (i.e. it's more important to be able to mute audio than to
seek). By computing these components before painting, we can more easily
allocate sections to the components in priority order, until the area
remaining has been depleted.
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.
