Allow dragging multiple files from a Dired buffer

Allow dragging multiple files from a Dired buffer

* doc/lispref/frames.texi (Drag and Drop): Document new function
`dnd-begin-drag-files'.
* lisp/dired.el (dired-mouse-drag-files): Update doc string.
(dired-map-over-marks): Accept a new value of ARG `marked',
meaning to not fall back to the current file if no marks were
found.
(dired-mouse-drag): Handle marked files in an intuitive way.

* lisp/dnd.el (dnd-last-dragged-remote-file): Allow list values
as well.
(dnd-remove-last-dragged-remote-file): Handle list values.
(dnd-begin-file-drag): Fix file name expansion.
(dnd-begin-drag-files): New function.
* lisp/select.el (xselect-convert-to-filename): Handle mutiple
files
(a vector of file names):.