GFile: add new g_file_measure_disk_usage() API

This is essentially the equivalent of 'du'.

This is currently only supported on local files.  gvfs will add support for the
interface later.

https://bugzilla.gnome.org/show_bug.cgi?id=704893
tests/: add gio-du

This is basically a minimally-featured 'du' equivalent to manually test
g_file_measure_disk_usage().

https://bugzilla.gnome.org/show_bug.cgi?id=704893