fixes #790

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

Fix compilation of PythonUtils.cpp with Visual Studio, and implement a more portable version of dirExists()

Have you tested your changes (if applicable)? If so, how?

No, as I don't have Windows or VS