Add user options to move point and scroll window during DND

Add user options to move point and scroll window during DND

* doc/emacs/frames.texi (Drag and Drop):
* etc/NEWS: Document new options 'dnd-scroll-margin' and
'dnd-indicate-insertion-point'.
* lisp/dnd.el (dnd-protocol-alist):
(dnd-open-remote-file-function):
(dnd-open-file-other-window): Add right group to defcustoms.
(dnd-scroll-margin, dnd-indicate-insertion-point): New user
options.
(dnd-handle-movement): New function.
* lisp/x-dnd.el (x-dnd-handle-xdnd):
(x-dnd-handle-motif): Call `dnd-handle-movement' when
appropriate.