With GJS' GDBus implementation, we get the invocation paramters as an
array if we declare a method as async.
This is bad and not consistent with what GJS does for synchronous
methods, but it's the way it is, and other classes in gnome-shell
implement this correctly by exploding the array into its components in
the method implementation, but not the screenshot methods.
Also, we're supposed to return a value using the provided invocation
object, not with a callback now, so do that.
https://bugzilla.gnome.org/show_bug.cgi?id=667662
7c108e267c