Support receiving XDS drops correctly

Support receiving XDS drops correctly

* etc/NEWS: Announce new feature.  It is not yet documented.
* lisp/x-dnd.el (x-dnd-known-types): Add XdndDirectSave0.
(x-dnd-direct-save-function): New defcustom.
(x-dnd-xdnd-to-action): Add `direct-save'.
(x-dnd-maybe-call-test-function): If XDS is present, use `direct-save'.
(x-dnd-find-type): New function.
(x-dnd-handle-xdnd): Handle XDS position and drop messages.
(x-dnd-handle-direct-save): Don't use local-file-uri if nil.
(x-dnd-save-direct): New function.
(x-dnd-handle-octet-stream-for-drop):
(x-dnd-handle-xds-drop): New functions.