Unify license to LGPL v2.1+
Summary:
Most of the code in here is using this already, some files however are
GPLv2-only.
For this to go in it needs explicitly approval from Laurent and Dan, who
wrote all the affected code.
Reviewers: mlaurent, dvratil
Reviewed By: mlaurent, dvratil
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D16110