cross-platform way to store application settings in python

[dhrogers]

We need some way to store application settings for pycinema. This appears to be a good option:

• https://github.com/platformdirs/platformdirs

There is a pyqt-specific way to do this as well, but I would like to keep this capability separate from pyqt, because pycinema can be run headless (without Qt).

@JonasLukasczyk, @EthanS94, @ollielo - any opinions on this approach?