Implement the Motif drag protocol

Implement the Motif drag protocol

* src/xterm.c (struct x_client_list_window): New field
`xm_protocol_style'.
(xm_top_level_enter_message, xm_drag_motion_message)
(xm_top_level_leave_message): New structures.
(xm_setup_drag_info, xm_send_top_level_enter_message)
(xm_send_drag_motion_message, xm_send_top_level_leave_message):
New functions.
(x_dnd_compute_toplevels): Compute `xm_protocol_style'.
(x_dnd_get_target_window_1, x_dnd_get_target_window): New
parameter `motif_out'.  Place the xm protocol style in it if
necessary.
(x_dnd_cleanup_drag_and_drop, x_dnd_begin_drag_and_drop)
(x_dnd_update_state, handle_one_xevent): Handle Motif drag
protocol messages.
(x_free_frame_resources): Cancel Motif drag protocol operations
correctly.