Add support for dragging text from Emacs to other programs

Add support for dragging text from Emacs to other programs

This still probably needs some more protection from
malfunctioning clients which delete windows at random, but I
don't know if that's a problem in practice.

* doc/emacs/frames.texi (Drag and Drop):
* doc/lispref/frames.texi (Drag and Drop): Document new
features.

* etc/NEWS: Announce new function `x-begin-drag' and new user
option `mouse-drag-and-drop-region-cross-program'.

* lisp/mouse.el (mouse-drag-and-drop-region-cross-program): New
user option.
(x-begin-drag): New variable declaration.
(mouse-drag-and-drop-region): If the mouse moves out of an Emacs
frame, begin a window system drag.
* lisp/x-dnd.el (x-dnd-handle-xdnd): Remove left-over debugging
code.

* src/xfns.c (Fx_set_mouse_absolute_pixel_position): Fix
indentation of opening paren.
(Fx_begin_drag): New function.
(syms_of_xfns): Define new subr.
* src/xselect.c (x_timestamp_for_selection): New function.

* src/xterm.c (X_DND_SUPPORTED_VERSION): New preprocessor
declaration.
(x_dnd_get_window_proto, x_dnd_send_enter, x_dnd_send_position)
(x_dnd_send_leave, x_dnd_send_drop, x_set_dnd_targets)
(x_dnd_begin_drag_and_drop): New functions.
(handle_one_xevent): Handle drag-and-drop motion and button
events when active.
(x_free_frame_resources): If f is the DND source, stop
drag-and-drop.
(x_term_init): Intern new atoms.
(syms_of_xterm): New symbol QXdndSelection.

* src/xterm.h (struct x_display_info): New atoms
Xatom_XdndAware, Xatom_XdndSelection, Xatom_XdndTypeList,
Xatom_XdndActionCopy, Xatom_XdndActionMove,
Xatom_XdndActionLink, Xatom_XdndActionAsk,
Xatom_XdndActionPrivate, Xatom_XdndActionList,
Xatom_XdndActionDescription, Xatom_XdndProxy, Xatom_XdndEnter,
Xatom_XdndPosition, Xatom_XdndStatus, Xatom_XdndLeave,
Xatom_XdndDrop, and Xatom_XdndFinished.