D27103: Convert license headers to SPDX markers

[Andreas Cord-Landwehr]

cordlandwehr added a task: T11550: Add SPDX License markers.

REPOSITORY
  R275 KItemModels

REVISION DETAIL
  https://phabricator.kde.org/D27103

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