Make project REUSE compatible

The change was performed as follows:
- it was checked that the COPYING.MIT license was present with the
  initial commit and such each contributor was awere of the license
- mail was around to all previous contributors to check this assumption
  (mail was not replied by Hannah and Sebastian); every reply was a
  confirmation of the assumed MIT licensing
- all copyright information were recovered from the respective Git
  commits
- reuse lint was to used to check the REUSE conformance

classes/cmake_kf5.bbclass

@@ -1,3 +1,9 @@
+# SPDX-FileCopyrightText: 2014 Johan Thelin <johan.thelin@pelagicore.com>
+# SPDX-FileCopyrightText: 2016 Sébastien Taylor <sebastien@au-zone.com>
+# SPDX-FileCopyrightText: 2017-2019 Volker Krause <vkrause@kde.org>
+#
+# SPDX-License-Identifier: MIT
+
 inherit cmake_qt5
 
 EXTRA_OECMAKE_class-native += " \