Handle mouse movement correctly during DND from one of our own frames

Handle mouse movement correctly during DND from one of our own frames

* lisp/dnd.el (dnd-handle-movement): Select the window specified
in posn.
* lisp/term/x-win.el (x-dnd-movement): New function.
(x-dnd-movement-function): Set it as the default.
* src/frame.c (delete_frame): Prevent deleting the drop source
frame.
* src/xterm.c (x_dnd_send_position): Set new mouse movement
flags if the target window is one of our own frames.
(x_dnd_begin_drag_and_drop): Call DND movement function whenever
appropriate.
(x_free_frame_resources): Remove useless code.
(syms_of_xterm): New defvar `x-dnd-movement-function'.
* src/xterm.h: Update prototypes.