qt: Remove obsolete QModelIndex::child()

The QModelIndex::child() member function is obsolete since Qt 5.12.

Author: hebasto

src/qt/receivecoinsdialog.cpp

@@ -261,7 +261,7 @@ void ReceiveCoinsDialog::copyColumnToClipboard(int column)
     if (!firstIndex.isValid()) {
         return;
     }
-    GUIUtil::setClipboard(model->getRecentRequestsTableModel()->data(firstIndex.child(firstIndex.row(), column), Qt::EditRole).toString());
+    GUIUtil::setClipboard(model->getRecentRequestsTableModel()->index(firstIndex.row(), column).data(Qt::EditRole).toString());
 }
 
 // context menu

src/qt/recentrequeststablemodel.h

@@ -76,7 +76,7 @@ class RecentRequestsTableModel: public QAbstractTableModel
     QVariant data(const QModelIndex &index, int role) const;
     bool setData(const QModelIndex &index, const QVariant &value, int role);
     QVariant headerData(int section, Qt::Orientation orientation, int role) const;
-    QModelIndex index(int row, int column, const QModelIndex &parent) const;
+    QModelIndex index(int row, int column, const QModelIndex &parent = QModelIndex()) const;
     bool removeRows(int row, int count, const QModelIndex &parent = QModelIndex());
     Qt::ItemFlags flags(const QModelIndex &index) const;
     /*@}*/