gio-tool-trash: Prevent recursion to speed up emptying trash

Emptying trash over `gio trash` is a bit slow in comparison to plain
`rm -r`. On my system, it took about 3 min to empty the trash with a
folder containing 600 000 files, which is not ideal as `rm -r` call
took just a few seconds. I found that `g_file_delete` is implemented
differently for locations provided by the trash backend. The trash
backend prevents modifications of trashed content thus the delete
operation is allowed only for the top-level files and folders. So it
is not necessary to recursive delete all files as the permission
denied error is returned anyway. Let's call `g_file_delete` only for
top-level items, which reduces the time necessary for emptying trash
from minutes to seconds...

See: nautilus#1589