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 += " \

classes/kauth.bbclass

@@ -1,4 +1,8 @@
 # Class for recipes using KAuth host tools
+#
+# SPDX-FileCopyrightText: 2019 Volker Krause <vkrause@kde.org>
+#
+# SPDX-License-Identifier: MIT
 
 DEPENDS += " \
     kauth \

classes/kconfig.bbclass

@@ -1,4 +1,8 @@
 # Class for recipes using kconfig host tools
+#
+# SPDX-FileCopyrightText: 2019 Volker Krause <vkrause@kde.org>
+#
+# SPDX-License-Identifier: MIT
 
 DEPENDS += " \
     kconfig \

classes/kcoreaddons.bbclass

@@ -1,4 +1,8 @@
 # Class for recipes using kcoreaddons host tools
+#
+# SPDX-FileCopyrightText: 2019 Volker Krause <vkrause@kde.org>
+#
+# SPDX-License-Identifier: MIT
 
 do_compile_prepend() {
     # desktoptojson needs to find installed service type files

classes/kdoctools.bbclass

@@ -1,4 +1,8 @@
 # Class for recipes using kdoctools to build their documentation
+#
+# SPDX-FileCopyrightText: 2019 Volker Krause <vkrause@kde.org>
+#
+# SPDX-License-Identifier: MIT
 
 do_compile_prepend() {
     # meinproc5 needs to find installed service type files

classes/ki18n.bbclass

@@ -1,4 +1,9 @@
 # Class for recipes using ki18n for internationalization
+#
+# SPDX-FileCopyrightText: 2019 Volker Krause <vkrause@kde.org>
+#
+# SPDX-License-Identifier: MIT
+
 inherit python3native
 inherit gettext
 

classes/reuse_license_checksums.bbclass

@@ -1,8 +1,9 @@
-# SPDX-FileCopyrightText: 2020 Andreas Cord-Landwehr <cordlandwehr@kde.org>
-# SPDX-License-Identifier: MIT
-#
 # Class for providing default checksum values based on licenses
 # installed according to REUSE specification
+#
+# SPDX-FileCopyrightText: 2020 Andreas Cord-Landwehr <cordlandwehr@kde.org>
+#
+# SPDX-License-Identifier: MIT
 
 do_populate_lic_prepend() {
     license = d.getVar('LICENSE') or ''