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