Destroy scenes in tests that were not destroyed
Destroy contexts that were not destroyed
Fix scene leak in CesiumWidget
Fixed leaked canvases
Fix incorrect URL to CesiumWidget.css.
Fix whitespace and typos.
Add HTTP method and data to loadWithXhr.

Also switch to an options object, since most of the options are optional.
Remove unnecessary override of loadWithXhr.
Fixes after review.
Fix extra comma in example.
Update CHANGES.md
Rename onDropError to dropError