Fix compliance with the XDND specification when dropping on a proxy

Fix compliance with the XDND specification when dropping on a proxy

* src/xterm.c (x_dnd_send_enter, x_dnd_send_position)
(x_dnd_send_leave, x_dnd_send_drop, x_dnd_do_drop): New
parameter `toplevel'.  Use it as the window in sent client
messages.
(x_dnd_cancel_dnd_early, x_dnd_cleanup_drag_and_drop)
(x_dnd_process_quit, x_dnd_update_state, handle_one_xevent)
(x_connection_closed): Give the right toplevel to the client
message sending functions.