It looks like this (and all of the widgets) now directly depend on CesiumWidget.css, but the css files for the individual widgets themselves do not include it.  As an open question, should CesiumWidget.css be the central place for shared CSS?  CesiumWidget itself has no buttons or UI, so wouldn't that bloat Cesium Widget-only apps?  Should we have a shared.css or something?

CC @shunter