Configuration path should have single source of truth

[jstsmthrgk]

Back when I made #2047 I only changed one of apparently four (I realized it because of #1437) places where the configuration path is being decided. It would make sense to just have one single place where the selection logic is run.

My suggestions would be to either:

• have a static function in PreferencesManager that computes the configuration path and returns it, or
• compute it in main.cpp and somehow store it in PreferencesManager (either as a preference or with its own getter/setter)

If I get told which way is prefered, I will implement it and make a pull request. (Also if wished I could directly incorporate #1437 in it.)