Reorganized DimLink Code

Moved the DimLink code out of NewDimData in order to reduce clutter
inside that class and made it a separate class, except for functions
that should only be available for NewDimData. Deleted IDimLink and
changed all references to it to use DimLink instead. DimLink is now an
abstract class, which achieves the same encapsulation and protection we
had before by having DimLink implement IDimLink from within NewDimData.
NewDimData has a new class inside, InnerDimLink, which provides it
access to special functions that would be dangerous to expose. This is
the same mechanism used to protect NewDimData's dangerous functions.
These changes are in preparation for adding more code for packet
handling.