Summary:
Convert license headers to SPDX statements and add
license files as required by REUSE specification.

Reviewers: cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: ognarb, cgiboudeaux, kde-frameworks-devel

Tags: #frameworks

Maniphest Tasks: T11550

Differential Revision: https://phabricator.kde.org/D27601