D23815: [KConfig] port away from deprecated methods in Qt 5.14

[David Faure]

dfaure added a reviewer: vkrause.

REPOSITORY
  R237 KConfig

REVISION DETAIL
  https://phabricator.kde.org/D23815

To: dfaure, mdawson, arichardson, vkrause
Cc: arojas, mlaurent, kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, 
bruns