GFile: Add g_file_delete_async()

This looks like it was stubbed out but not implemented; the vtable
entry dates to commit 37813437 which
is just alex's initial merge of gio into glib.

I was working on some code that wants an asynchronous rm -rf
equivalent, and so yeah, this is desirable.

https://bugzilla.gnome.org/show_bug.cgi?id=680760