REUSE: fix credits for 3rd party sources

Some Calamares source files incorporate material from
3rd parties (unlike the 3rdparty/ dir, which is basically-
unchanged 3rd party source). Tidy up the FileCopyrightText
lines for those sources.

This is not an exhaustive effort.

src/libcalamaresui/utils/ImageRegistry.cpp

@@ -1,28 +1,11 @@
 /* === This file is part of Calamares - <https://github.com/calamares> ===
  *
+ *   SPDX-FileCopyrightText: 2012 Christian Muehlhaeuser <muesli@tomahawk-player.org>
+ *   SPDX-FileCopyrightText: 2019, Adriaan de Groot <groot@kde.org>
  *   SPDX-License-Identifier: GPL-3.0-or-later
- *   License-Filename: LICENSES/GPLv3+-ImageRegistry
  *
- *   Copyright 2019, Adriaan de Groot <groot@kde.org>
  */
 
-/*
- *   Copyright 2012, Christian Muehlhaeuser <muesli@tomahawk-player.org>
-
-    This program is free software: you can redistribute it and/or modify
-    it under the terms of the GNU General Public License as published by
-    the Free Software Foundation, either version 3 of the License, or
-    (at your option) any later version.
-
-    This program is distributed in the hope that it will be useful,
-    but WITHOUT ANY WARRANTY; without even the implied warranty of
-    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-    GNU General Public License for more details.
-
-    You should have received a copy of the GNU General Public License
-    along with this program.  If not, see <http://www.gnu.org/licenses/>.
-*/
-
 #include "ImageRegistry.h"
 
 #include <QIcon>

src/modules/users/CheckPWQuality.cpp

@@ -1,6 +1,11 @@
 /* === This file is part of Calamares - <https://github.com/calamares> ===
  *
- *   Copyright 2018, Adriaan de Groot <groot@kde.org>
+ *   SPDX-FileCopyrightText: 2018 Adriaan de Groot <groot@kde.org>
+ *   SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   Contains strings from libpwquality under the terms of the
+ *   GPL-3.0-or-later (libpwquality is BSD-3-clause or GPL-2.0-or-later,
+ *   so we pick GPL-3.0-or-later).
  *
  *   Calamares is free software: you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by

src/qml/calamares/slideshow/Presentation.qml

@@ -1,19 +1,22 @@
 /* === This file is part of Calamares - <https://github.com/calamares> ===
  *
- *   Copyright 2017, Adriaan de Groot <groot@kde.org>
+ *   SPDX-FileCopyrightText: 2017 Adriaan de Groot <groot@kde.org>
+ *   SPDX-FileCopyrightText: 2016 The Qt Company Ltd.
+ *   SPDX-License-Identifier: LGPL-2.1-only
+ *
+ *   2017, Adriaan de Groot <groot@kde.org>
  *     - added looping, keys-instead-of-shortcut
- *   Copyright 2018, Adriaan de Groot <groot@kde.org>
+ *   2018, Adriaan de Groot <groot@kde.org>
  *     - make looping a property, drop the 'c' fade-key
  *     - drop navigation through entering a slide number
  *       (this and the 'c' key make sense in a *presentation*
  *       slideshow, not in a passive slideshow like Calamares)
  *     - remove quit key
- *   Copyright 2019, Adriaan de Groot <groot@kde.org>
+ *   2019, Adriaan de Groot <groot@kde.org>
  *     - Support "V2" loading
  *     - Disable shortcuts until the content is visible in Calamares
- *
- *   SPDX-License-Identifier: LGPL-2.1-only
- *   License-Filename: LICENSES/LGPLv2.1-Presentation
+ *   2020, Adriaan de Groot <groot@kde.org>
+ *     - Updated to SPDX headers
  */
 
 /****************************************************************************

src/qml/calamares/slideshow/Slide.qml

@@ -1,7 +1,7 @@
 /* === This file is part of Calamares - <https://github.com/calamares> ===
  *
+ *   SPDX-FileCopyrightText: 2012 Digia Plc and/or its subsidiary(-ies).
  *   SPDX-License-Identifier: LGPL-2.1-only
- *   License-Filename: LICENSES/LGPLv2.1-Presentation
  */
 
 /****************************************************************************