GLocalFile: use GDir for g_file_measure_disk_usage

GLocalFile: use GDir for g_file_measure_disk_usage

It turns out that although dirent is available on mingw32 (where the
code was originally tested), it is not usable from MSVC.

Avoid portability problems by just using GDir.

Also, be careful about ensuring that we utf8-format filenames in our
error messages, and leave out the "file://" component since the strings
we're displaying are not URIs (and we don't want to make them URIs since
the extra escaping would reduce legibility).

Thanks to Chun-wei Fan <fanchunwei@src.gnome.org> for portions of this
patch and for reviews.

https://bugzilla.gnome.org/show_bug.cgi?id=707787