Add easier-to-use interfaces for initiating drag-and-drop

Add easier-to-use interfaces for initiating drag-and-drop

The previous interface required that users know intricacies of
the data types used to transfer data on each platform Emacs
supports.

* doc/lispref/frames.texi (Drag and Drop): Document new
functions.
* lisp/dired.el (dired-last-dragged-remote-file)
(dired-remove-last-dragged-local-file): Delete functions.
(dired-mouse-drag): Use `dnd-begin-file-drag'.
* lisp/dnd.el (dnd-last-dragged-remote-file)
(dnd-remove-last-dragged-remote-file): New variables and
functions.
(dnd-begin-text-drag, dnd-begin-file-drag): New functions.
* src/xterm.c (x_dnd_begin_drag_and_drop): Add porting note.