Avoid ClientMessage overhead when dragging stuff to other frames

Avoid ClientMessage overhead when dragging stuff to other frames

* lisp/dired.el (dired-mouse-drag): Handle correctly dragging
from dired buffers in nonselected windows.
* lisp/x-dnd.el (x-dnd-handle-drag-n-drop-event): Understand new
client message type.
* src/xterm.c (x_dnd_send_enter, x_dnd_send_position)
(x_dnd_send_leave): Ignore if window is the top window of a
frame.
(x_dnd_send_drop): Send special DND event in that case.