Handle alternate actions for Motif drop targets

Handle alternate actions for Motif drop targets

* src/xterm.c (xm_side_effect_from_action): Handle
`XdndActionAsk'.
(xm_operations_from_actions): New function.
(xm_send_top_level_leave_message, x_dnd_cleanup_drag_and_drop)
(x_dnd_update_state, handle_one_xevent, x_connection_closed):
Use those actions when building a side effect.
(x_dnd_begin_drag_and_drop): Keep track of the set of
alternative drag-and-drop actions.