Standardize CSS names

So far, our widgets have been playing fast and loose with the CSS
namespace.  This standardizes everything to avoid potential
collisions.  The new rule is everything should follow the
cesium-<item>-<cssClassName> convention.

Examples:
cesium-lighter
cesium-widget-[xxx]
cesium-timeline-[xxx]
cesium-animation-[xxx]
cesium-fullscreen-[xxx]

I'll also update the coding conventions once this is in.