Prototype data source plugins
Ongoing data source prototyping.
Merge branch 'master' into dataSources
Ongoing data source prototyping.
Merge branch 'master' into dataSources
temporary fix for #issue746.
Doc and clean up for new data source work.

Still needs specs.

I also reverted my changes to CesiumViewerWidget since I don't want to break the API just yet and it will be superceded by the "in progress" new Viewer widget.
When reverting the file, I forgot to maintain last new line.
Data source display cleanup.
Make CzmlDataSource asynchronous functions return promises.
Merge branch 'master' into dataSources
Merge remote-tracking branch 'origin/master' into issue746
Added clipToWindowCoordinates
Added fromElements
Updated CHANGES.md
Added wgs84ToWindowCoordinates
Removed Polygon.computeScreenSpacePosition
Cleaned up Billboard actual position
Added doc
Added missing doc