livesync: Cleanups and minor fixes

A set of minor cleanups and fixes for the livesync element; not important to backport.

• livesync: Move single segment prop
  Keep it with the settings, not after the stats.

• livesync: Extract audio_info_from_caps
  And adjust it slightly so it never panics.

• livesync: Extract LiveSync::flow_error
  And add details so it behaves more like the GST_ELEMENT_FLOW_ERROR macro.

• livesync: Fix log message capitalization

• livesync: Improve formatting
  Move some code around to make it a bit more readable. No change in behavior.

• livesync: Document State's fields