The `DynamicCellSet` class is now marked as deprecated (as is the header
that contains it), and all non-deprecated code is moved to its
`UnknownCellSet` replacement.
Also added a deprecation warning for the VariantArrayHandle.h header
file and deleted a couple inappropriate uses of it.
The VS compiler gives a warning when you override a deprecated method.
Changed the StartScene and EndScene methods in Mapper to be non-virtual
with empty implementations. Deleted the corresponding methods from all
subclasses.