[libcalamaresui] Reduce 0-for-nullptr warnings in 3rd party code

- replace = 0 with = nullptr to reduce warnings; not a meaningful
   or copyrightable change.

src/libcalamaresui/utils/qjsonitem.h

@@ -13,7 +13,7 @@
 class QJsonTreeItem
 {
 public:
-    QJsonTreeItem(QJsonTreeItem * parent = 0);
+    QJsonTreeItem(QJsonTreeItem * parent = nullptr);
     ~QJsonTreeItem();
     void appendChild(QJsonTreeItem * item);
     QJsonTreeItem *child(int row);
@@ -28,7 +28,7 @@ public:
     QJsonValue::Type type() const;
 
 
-    static QJsonTreeItem* load(const QJsonValue& value, QJsonTreeItem * parent = 0);
+    static QJsonTreeItem* load(const QJsonValue& value, QJsonTreeItem * parent = nullptr);
 
 protected:
 

src/libcalamaresui/utils/qjsonmodel.h

@@ -16,7 +16,7 @@ class QJsonModel : public QAbstractItemModel
 {
     Q_OBJECT
 public:
-    explicit QJsonModel(QObject *parent = 0);
+    explicit QJsonModel(QObject *parent = nullptr);
     ~QJsonModel();
     bool load(const QString& fileName);
     bool load(QIODevice * device);

src/libcalamaresui/widgets/waitingspinnerwidget.h

@@ -37,7 +37,7 @@ class WaitingSpinnerWidget : public QWidget {
 public:
     /*! Constructor for "standard" widget behaviour - use this
    * constructor if you wish to, e.g. embed your widget in another. */
-    WaitingSpinnerWidget(QWidget *parent = 0,
+    WaitingSpinnerWidget(QWidget *parent = nullptr,
                          bool centerOnParent = true,
                          bool disableParentWhenSpinning = true);
 
@@ -47,7 +47,7 @@ public:
    * QtWaitingSpinner automatically centres itself on it, if not,
    * "centreOnParent" is ignored. */
     WaitingSpinnerWidget(Qt::WindowModality modality,
-                         QWidget *parent = 0,
+                         QWidget *parent = nullptr,
                          bool centerOnParent = true,
                          bool disableParentWhenSpinning = true);