D26184: QHash::unite is deprecated in qt5.15. We need to port to QMultiHash.But QMultiHash::unite was added in qt5.13

[David Faure]

dfaure accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R276 KItemViews

BRANCH
  port_qhash_deprecated (branched from master)

REVISION DETAIL
  https://phabricator.kde.org/D26184

To: mlaurent, dfaure
Cc: kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns