Add preprocessor macros for each wrapper language.

These new macros can be used to detect when the headers are being parsed by one of the wrapper tools, rather than by the C++ compiler:

__VTK_WRAP_PYTHON__
__VTK_WRAP_TCL__
__VTK_WRAP_JAVA__

Also, the old __WRAP__ macro should no longer be used, it has been replaced with __VTK_WRAP__.