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As discussed with dscharrer on IRC, the idea would be to give the possibility to add a custom Arx Fatalis install location to the data path, probably storing it as a "custom_data_dir" variable in cfg.ini.

There could be a --set-custom-data-dir switch to define it using the CLI, and also an opt-in checkbox ("Use this directory as custom data path") when using arx-install-data to install the game data to non-default path or to verify an existing install.

This would make it possible e.g. for Windows users who installed Arx Fatalis 1.21 to point Arx Libertatis to their installation folder, and play the game right away without having to copy the data files to the Arx Libertatis default path.