Include Right Python Header for OS

Conditions the Python header included on the OS, since OS X puts it
in a different place than Linux.

Changes:

 * combine_perfReses_dialog.cpp
Add Plugin Directory to Python Path at Runtime

Adds code to combine_perfReses_dialog.cpp to modify the PYTHONPATH /
sys.path so that the Python runtime will search for the operator plugins
in the directory where they are installed, eliminating the necessity to
set the PYTHONPATH variable if the software has been installed.

Changes:

 * CMakeLists.txt: Generate a header containing the path to the python
   plugins.
 * operator_python_module_conf.h.in: Input file for header generation.
 * combine_perfReses_dialog.cpp: Include the generated header with the
   path the operator modules, use the path included in it to modify
   the Python path.