Licensing: re-import GPLv3 version of QJsonItem

- include full license headers,
 - copied from
    - repo: https://github.com/dridk/QJsonmodel
    - rev:  fbd4fb3b1c1311f69cd8ada9bbb7a89fd6fe171e

src/libcalamaresui/utils/qjsonmodel.h

@@ -1,23 +1,3 @@
-/* === This file is part of Calamares - <http://github.com/calamares> ===
- *
- *   Originally from QJsonModel <https://github.com/dridk/QJsonmodel>
- *   Copyright 2015, Sacha Schutz <sacha@labsquare.org>
- *
- *   Calamares 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.
- *
- *   Calamares 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 Calamares. If not, see <http://www.gnu.org/licenses/>.
- */
-
-
 #ifndef QJSONMODEL_H
 #define QJSONMODEL_H
 
@@ -30,8 +10,7 @@ class QJsonModel : public QAbstractItemModel
 {
     Q_OBJECT
 public:
-    explicit QJsonModel(QObject *parent = nullptr);
-    virtual ~QJsonModel();
+    explicit QJsonModel(QObject *parent = 0);
     bool load(const QString& fileName);
     bool load(QIODevice * device);
     bool loadJson(const QByteArray& json);