Implement missing parts of the Motif drag and drop protocol

Implement missing parts of the Motif drag and drop protocol

* src/xterm.c (xm_drop_start_reply): New structure.
(xm_get_drag_window): Don't grab the server since this leads to
weird freezes when creating the drag window.
(xm_read_drop_start_reply): New function.
(x_dnd_begin_drag_and_drop): Set Motif finish flag to 0.
(handle_one_xevent): When starting a motif drop, set the finish
flag to 1.  When the receiver replies to our drop message, set
the finish flag to 2 if the drop was accepted, and only clear
the waiting for finish flag when a selection request for
XmTRANSFER_SUCCESS or XmTRANSFER_FAILURE arrives.
(x_term_init): New atoms.
* src/xterm.h (struct x_display_info): New atoms.