License maintenance: GPLv2 -> GPLv2+

Summary:
This is only the first batch for mailcommon, and entirely based on
approval from https://techbase.kde.org/Projects/KDE_Relicensing.

Reviewers: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D16808