Make drag-and-drop wheel movement work locally too on X

Make drag-and-drop wheel movement work locally too on X

This is provided by the XDND protocol, so it isn't fair for it
to not work with local drag-and-drop.

* lisp/x-dnd.el (x-dnd-note-wheel-movement): New function.  Set
it as the `x-dnd-wheel-function'.
* src/xterm.c (x_dnd_cleanup_drag_and_drop): Clear new flags.
(x_dnd_note_self_wheel): New function.  Set some flags.
(x_dnd_process_quit, x_dnd_begin_drag_and_drop, handle_one_xevent)
(x_connection_closed, x_delete_terminal, mark_xterm): Handle and
set new wheel movement flags
(syms_of_xterm): New variable `x-dnd-wheel-function'.