Optional new names for Mapscript classes

[sgillies@…]

Am going to support an experimental change in Mapscript class names 
that is controlled with a preprocessor symbol NEXT_GENERATION_CLASSES.

Will use it like

  swig -python -shadow -DNEXT_GENERATION_CLASSES mapscript.i

And if the symbol is defined we will rename

  layeroObj -> Layer
  mapObj -> Map

et cetera.

I'll also write a new layerObj.getShape method that is like the PHP-Mapscript
method which will be used if we've defined NEXT_GENERATION_CLASSES.

Nothing else changes so it won't break compatibility with the general SWIG
documentation.

[sgillies@…]

Have added directives in mapscript.i that do the renaming.  New test cases
for the next generation classes pass.

[sgillies@…]

This feature is now controlled by the NEXT_GENERATION_NAMES symbol.

[sgillies@…]

Done, and is working.

[sgillies@…]

Dropping this option from mapscript.  Will not be an option in 4.4.