XtGetSelectionRequest
may only be called from within an XtConvertSelectionProc procedure
and returns a pointer to the SelectionRequest event that caused the conversion
procedure to be invoked. Request_id specifies a unique id for the individual
request in the case that multiple incremental transfers are outstanding.
For atomic transfers, request_id must be specified as NULL. If no SelectionRequest
event is being processed for the specified widget, selection, and request_id,
XtGetSelectionRequest returns NULL.