Update design doc with step-start docs.
Add eos field to step done message
when stepping in reverse, update the segment time field.
Flush out the current step when we are flushing.
Update framestep document, we want to pass the flush flag in the step-done
message.
Add flush flag to the gstmessage.
Update examples to use the new step-done message api.
Implement framestep with playback rates < 0.0 too.
