Widget cleanup

1. The first parameter to each widget is now uniformly named and documented.  It can either be an element or string.
2. Each widget exposes a public `container` property, which is the element that was passed into the constructor.
3. `cesium-darker` is now the default theme, deleted `darker.css` files and replaced them with `lighter.css`  For each of use, there is now a `lighter.css` in the Widgets directory which just includes all other `lighter.css` files.

I plan on continuing widget cleanup, but decided it was better to make small, incremental pull requests rather than shoving everything together.