GIO_EXTRA_MODULES option as a list of paths

The environment variable `GIO_EXTRA_MODULES` can be a list of paths. It does not have to be a single path.

Currently it is defined only in `xfce.nix` and `gnome3.nix` as strings.

This change will make it possible to merge definitions from different modules. Without the change a merge error occurs because options of type string cannot be merged.