Rename DynamicScene to DataSources

As discussed in #1727, this renames the `DynamicScene` layer to `DataSources` it also renames many related objects and files.  I have to update CHANGES, but this is otherwise ready for review.  Due to the large scale renames, I would prefer to get this into master ASAP.

Fixed #1727 

I think that's a good idea; same for `selectedObject`.  Both will be more clear.