Wrap more classes for python.
This makes the python wrappers ignore WRAP_EXCLUDE, and instead use a new property WRAP_EXCLUDE_PYTHON that excludes fewer classes. The WRAP_SPECIAL flag, which used to act as a whitelist for python, has been removed.
Because this change causes classes to be wrapped in python by default, some third-party VTK packages might break until they modify their own CMakeLists.txt files to add WRAP_EXCLUDE_PYTHON where necessary.