Improve PYTHONPATH management per project

[albilu]

After standard project generation, I added the func1 function to the main.py file.
I then generated a standard test unit file. After these operations, the unit tests worked.
Next, I wanted to test the func1 function. I added the following code:
from main import func1
Unfortunately, this operation fails because the PYTHONPATH environment variable
contains the /.../Project/tests directory but not the /.../Project/ directory.
I solved this problem by adding the appropriate PYTHONPATH variable in the global
configuration NetBeans/Options/Python/Environment Variables but this is not
a good solution.

Can this be set somewhere just for a specific project? Maybe in the
nbproject/project.properties file?

Originally posted by @DKnoto in #31