Add wrapper script for Python on windows.

It is copied to the build config directopry and checks whether
the release/debug version of Python should be called depending on the
directory contents. Refs #6903
Use python wrapper in Qt assistant on windows.

This sorts out the correct call between release & debug python.
Refs #6903
Fix win_python wrapper paths and raise an error if nothing if found.

Refs #6903
Merge remote-tracking branch 'origin/feature/6903_qtassist_win_debug'