KDAV: Convert license headers to SPDX identifiers
Summary:
Note that this commit fixes ambiguous statements, which were clarified
in https://mail.kde.org/pipermail/kde-pim/2019-September/042912.html
Add license texts as required by REUSE
Reviewers: vkrause
Reviewed By: vkrause
Subscribers: kde-pim
Tags: KDE PIM
Maniphest Tasks: T11550
Differential Revision: https://phabricator.kde.org/D27571