Switch to RegisterUnitEvent for raid units

[cont1nuity]

This is a rudimentary implementation to switch from RegisterEvent to RegisterUnitEvent (with a fallback for unset unit) for all UNIT_* events.

This ensures that the events only are processed for the actual displayed unit and not any stray events are processed.
Depending on unit checks down the update line, this should massively improve performance in larger groups and more nameplates on screen, as those would fire global UNIT_ events as well.

In this implementation, the displayedUnit is not set on the first registry, leading to unregister and re-register of the events.
I did not spend much time in finding the full setup flow, so this can likely be improved.