Implement Motif drop protocol

Implement Motif drop protocol

This is the second most widely implemented drag-and-drop
protocol on X Windows, but seems to have some unsolvable
problems (i.e. stuff will keep accumulating in the drag window
as long the target lists keep changing.)  The implementation is
not yet complete and doesn't work with some programs.

* lisp/select.el (xselect-convert-xm-special): New functions.
(selection-converter-alist): Add new converters.
* lisp/x-dnd.el (x-dnd-handle-motif): Ignore messages sent by
the receiver.
* src/xterm.c (xm_targets_table_byte_order): New enum;
(SWAPCARD32, SWAPCARD16): New macros.
(xm_targets_table_rec, xm_drop_start_message)
(xm_drag_initiator_info, xm_drag_receiver_info): New structures.
(XM_DRAG_SIDE_EFFECT, xm_read_targets_table_header)
(xm_read_targets_table_rec, xm_find_targets_table_idx)
(x_atoms_compare, xm_write_targets_table)
(xm_write_drag_initiator_info, xm_get_drag_window)
(xm_setup_dnd_targets, xm_send_drop_message)
(xm_read_drag_receiver_info): New functions.
(x_dnd_compute_toplevels): Correctly free some temp data.
(x_dnd_get_window_proxy, x_dnd_get_window_proto)
(x_set_frame_alpha): Likewise.
(handle_one_xevent): If the window has no XDND proto but has
motif drag receiver data, send a motif drop protocol request.
(x_term_init): New atoms for Motif DND support.
* src/xterm.h (struct x_display_info): Add new atoms.