Returns a unique identifier for a message based on the context of the message, context_description.
Any messages added to a statusbar's stack must specify a context id that is used to uniquely identify the source of a message. This context id can be generated by get_context_id() , given a message, context_description describing the context of the message. Note that messages are stored in a stack, and when choosing which message to display, the stack structure is adhered to, regardless of the context identifier of a message.