Clean up text/uri-list mess inside the Dired drag-and-drop code

Clean up text/uri-list mess inside the Dired drag-and-drop code

* doc/lispref/frames.texi (Window System Selections):
* etc/NEWS: Document new changes to `gui-get-selection'.
* lisp/dired.el (dired-mouse-drag): Specify text/uri-list value
explicitly.
* lisp/select.el (gui-set-selection): Update doc string.
(xselect-convert-to-text-uri-list): Update to handle either a
single URL (as a string) or a vector of URLs, instead of file
names.
(xselect-uri-list-available-p): Likewise.
* src/xselect.c (x_get_local_selection): Look in tem's text
properties (if it is a string) for a local value before using
tem itself.