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