Better handle drag-and-drop from one Emacs frame to another

Better handle drag-and-drop from one Emacs frame to another

* doc/lispref/frames.texi (Drag and Drop): Document new
parameter `return-frame' to `x-begin-drag'.
* lisp/mouse.el (mouse-drag-and-drop-region): Utilize new
feature.

* src/xfns.c (Fx_begin_drag): New parameter `return-frame'.
* src/xterm.c (x_dnd_begin_drag_and_drop): New parameter
return_frame_p.
(handle_one_xevent): Set new flags and return frame whenever
appropriate.
* src/xterm.h: Update prototypes.