Use _NET_CLIENT_LIST_STACKING to optimize drag and drop window discovery

Use _NET_CLIENT_LIST_STACKING to optimize drag and drop window discovery

* src/xterm.c (struct x_client_list_window): New struct.
(x_dnd_free_toplevels, x_dnd_compute_toplevels)
(x_dnd_get_target_window_1): New functions.
(x_dnd_get_target_window): Search in the toplevel list if it
exists.
(x_dnd_cleanup_drag_and_drop): Clean up toplevel list.
(x_dnd_begin_drag_and_drop): Compute toplevel list if the
window manager supports it.
(handle_one_xevent): Update the toplevel list if prudent.