Add support for dragging text onto windows that don't support any DND protocol

Add support for dragging text onto windows that don't support any DND protocol

* src/xselect.c (x_own_selection): Export function.
(x_handle_selection_request): Handle selection requests to the
pending DND time.
(x_set_pending_dnd_time): New function.

* src/xterm.c (x_dnd_send_unsupported_drop): New function.
Implement according to the suggestions in the XDND protocol
specification.
(x_dnd_get_target_window): Return the toplevel window as well.
(x_dnd_cleanup_drag_and_drop, x_dnd_begin_drag_and_drop)
(x_dnd_update_state, handle_one_xevent): Send unsupported drops
and use last seen toplevel instead of target for `return_frame'.
* src/xterm.h: Update prototypes.