Enable DND handlers to receive more than one URI at a time

Enable DND handlers to receive more than one URI at a time

* doc/lispref/frames.texi (Drag and Drop): Illustrate the effect
of the dnd-multiple-handler property and how convergent handlers
are reconciled.

* etc/NEWS (Lisp Changes in Emacs 30.1): Announce this change.

* lisp/dnd.el (dnd-protocol-alist): Bring doc string up to date.
(dnd-handle-one-url): Obsolete this function.
(dnd-handle-multiple-urls): New function.

* lisp/pgtk-dnd.el (pgtk-dnd-handle-uri-list)
(pgtk-dnd-handle-file-name):

* lisp/term/android-win.el (android-handle-dnd-event):

* lisp/term/haiku-win.el (haiku-drag-and-drop):

* lisp/term/ns-win.el (ns-drag-n-drop):

* lisp/term/w32-win.el (w32-handle-dropped-file):

* lisp/x-dnd.el (x-dnd-handle-uri-list, x-dnd-handle-file-name):
Reimplement in terms of `dnd-handle-multiple-uris'.

* lisp/term/pgtk-win.el (pgtk-drag-n-drop)
(pgtk-drag-n-drop-other-frame, pgtk-drag-n-drop-as-text): Efface
detritus that remained after the removal of the old PGTK drag
and drop implementation.

* test/lisp/dnd-tests.el (ert-x, dnd-tests-list-1)
(dnd-tests-list-2, dnd-tests-list-3, dnd-tests-list-4)
(dnd-tests-local-file-function, dnd-tests-remote-file-function)
(dnd-tests-http-scheme-function, dnd-tests-browse-url-handler)
(dnd-tests-receive-multiple-urls): New tests.