Include a PYTHONPATH extension allowing for user-space packages

[freakboy3742]

What is the problem or limitation you are having?

The stub app currently sets a PYTHONPATH that includes the app and app_packages paths, plus the stdlib.

These paths are all inside the application bundle, which the user may not have access to modify.

If a user wants to add additional files to the PYTHONPATH (e.g., if you were building a "Jupyter Notebook" app, and wanted the user to be able to access user-installed packages), you can't install that code into app or app_packages.

Describe the solution you'd like

The stub app should include an XDG compliant user path as part of the standard app path. This is probably:

~/.local/share/<app name>/user_packages

Describe alternatives you've considered

Don't allow user-installed packages.

Additional context

See beeware/briefcase-linux-system-template#3 and beeware/briefcase-linux-appimage-template#30 for analogous problems.