CZML and Widgets roadmap

This was previously part of the [older roadmap on the wiki](https://github.com/AnalyticalGraphicsInc/cesium/wiki/Roadmap). I moved the items here without changing them. They are a few years old.

When someone more familiar with these has time, can you please update this and perhaps split into separate CZML and Widgets roadmaps? Not a high priority.

* CZML
* [CZML External links](https://github.com/AnalyticalGraphicsInc/cesium/wiki/External-links)
* GeoServer CZML writer
* [CZML Path/History visualization](https://github.com/AnalyticalGraphicsInc/cesium/wiki/CZML-History-visualization-details)
* Z-ordering - [czml-writer issue](https://github.com/AnalyticalGraphicsInc/czml-writer/issues/20)
* Buffer availability and "buffering" behavior
* Layers
* Improved materials support
* Custom axes & reference frames
* Imagery and terrain sources
* Camera/Tours
* Arbitrary link support (# and $ in CZML spec)
* CZML defined user interfaces, possibly with markdown
* Model support, once Cesium support exists
* Human readable messages/events raised in CZML
* Widgets
* Data Source Browser widget ([branch](https://github.com/AnalyticalGraphicsInc/cesium/compare/master...dataSourceBrowser))
* Navigation widget.
* Improved TimeLine widget
* Standard Cesium toolbar

Review the older wiki roadmap, the linked CZML External links and CZML History visualization pages, the czml-writer z-ordering issue, and the Data Source Browser comparison branch. Separate the listed CZML and Widgets items into current roadmaps, verify which remain relevant, and document the updated priorities and scope.

Written by the indexing model from the issue text.