1) use SCH_COMPONENT::GetRef(), GetValue() and GetFootprint() when
instance-specific info is needed
2) update UpdateAllScreenReferences() to handle value and footprint.
3) BACKANNO is CvPcb's handler, not back annotation's handler. Which
means it needs GUI behaviour, not back annotation behaviour.
Fixes https://gitlab.com/kicad/code/kicad/issues/5520