C API users can keep dealing with the implicit equivalence of
GdkFileList and GSList, but language bindings have no idea that one type
is another, and none of them exposes GSList as a type anyway, so they
will need a way to construct a GdkFileList.

Instead of making GdkFileList mutable, and re-implement GSList, we only
provide a constructors pair that lets you create a GdkFileList from a
linked list or from an array.