License maintenance: GPLv2 -> GPLv2+

Summary:
This is based on https://techbase.kde.org/Projects/KDE_Relicensing for
the individual authors. The only one not found on there is KDAB (hereby
approved).

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D16136