Allow dragging files from Dired to other programs

Allow dragging files from Dired to other programs

* etc/NEWS: Announce new user option `dired-mouse-drag-files'.
* lisp/dired.el (dired-mouse-drag-files): New user option.
(dired-mouse-drag): New command.
(dired-mouse-drag-files-map): New variable.
(dired-insert-set-properties): Add additional keymap if mouse
dragging is enabled.

* lisp/select.el (xselect-convert-to-targets): Handle new
form of selection converters.
(xselect-convert-to-username):
(xselect-convert-to-text-uri-list):
(xselect-uri-list-available-p): New functions.
(selection-converter-alist): Add them as selection converters.

* src/xselect.c (x_get_local_selection): Handle new form of
selection converters.
(syms_of_xselect): Update doc strings.