KCalendarCore: Convert license statements to SPDX markers
Summary:
Convert license statements to SPDX markers and
add license files as required by REUSE specification.
Reviewers: cgiboudeaux
Reviewed By: cgiboudeaux
Subscribers: cgiboudeaux, kde-pim
Tags: KDE PIM
Maniphest Tasks: T11550
Differential Revision: https://phabricator.kde.org/D27361