D23103: Add decibel power units (dBW and multiples)

[Ismael Asensio]

iasensio added a reviewer: broulik.

REPOSITORY
  R292 KUnitConversion

REVISION DETAIL
  https://phabricator.kde.org/D23103

To: iasensio, broulik
Cc: kde-frameworks-devel, LeGast00n, michaelh, ngraham, bruns