Prototype data source plugins
Ongoing data source prototyping.
Merge branch 'master' into dataSources
Ongoing data source prototyping.
Merge branch 'master' into dataSources
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I also reverted my changes to CesiumViewerWidget since I don't want to break the API just yet and it will be superceded by the "in progress" new Viewer widget.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 branch 'master' into dataSources
Specs for DataSourceCollection
Merge branch 'master' into dataSources
Specs for CzmlDataSource
Some basic DataSourceDisplay specs
Clean up and finish data source specs.
Minor doc cleanup.
Merge branch 'master' into dataSources
Merge branch 'master' into dataSources
Changes after review.