event: New option for live flushing

When dealing with flushes and live sources (i.e. that return GST_STATE_CHANGE_NO_PREROLL), the states of sinks would be lost resulting in clocks not advancing until those sinks were prerolled. Sadly gstbin (and gstpipeline) no longer have the information that there were no-preroll sources in the pipeline and that therefore prerolling should prevent staying in PLAYING.

As a workaround for this, add the possibility for live sources to be able to indicate that the FLUSH_START should not cause a lost of state.