Cesium UI and API

Hi guys,

A branch called “playbackTweaks” (pull request #501) has some re-architecting by @mramato with intentions of becoming the new standard for UI interactions with Cesium.

In particular, a new “Widgets/Command” class has been introduced, allowing API representation of commands issued by widget buttons, and unit-testing of those commands.

My description here is probably not doing it justice. But if anyone else is interested in the design of how future UI will interact with Cesium, I recommend you take a close look at what’s developing on this branch. Thanks,