nautilus_file_get_default_sort_type() has a misleading name. It doesn't
actually get the default sort type for any folder, but only for certain
special folders.

For regular folders, the default comes from GSettings keys, but this is
implemented in the views instead, with a lot of reduncant code.

So, let's do that in nautilus_file_get_default_sort_type() directly,
and clearly document it.