Kirigami: Convert license headers to SPDX markers
Summary:
Convert license statements to SPDX markers and
add license files as required by REUSE specification.
Reviewers: mart, jriddell
Reviewed By: mart
Subscribers: plasma-devel
Tags: Kirigami
Maniphest Tasks: T11550
Differential Revision: https://phabricator.kde.org/D27364