KGuiAddons: Convert license headers to SPDX statements
Summary:
Convert license statements to SPDX and add license text files in LICENSES
folder as required by the REUSE specification.
License clarification of kguiaddons/tests/openurltest.cpp given in:
https://mail.kde.org/pipermail/kde-frameworks-devel/2020-February/103423.html
Reviewers: cgiboudeaux
Reviewed By: cgiboudeaux
Subscribers: cgiboudeaux, kde-frameworks-devel
Tags: Frameworks
Maniphest Tasks: T11550
Differential Revision: https://phabricator.kde.org/D27297