Make C++11 mandatory.

src/gui/group/GroupModel.cpp

@@ -28,7 +28,7 @@
 
 GroupModel::GroupModel(Database* db, QObject* parent)
     : QAbstractItemModel(parent)
-    , m_db(Q_NULLPTR)
+    , m_db(nullptr)
 {
     changeDatabase(db);
 }
@@ -329,7 +329,7 @@ QStringList GroupModel::mimeTypes() const
 QMimeData* GroupModel::mimeData(const QModelIndexList& indexes) const
 {
     if (indexes.isEmpty()) {
-        return Q_NULLPTR;
+        return nullptr;
     }
 
     QMimeData* data = new QMimeData();
@@ -354,7 +354,7 @@ QMimeData* GroupModel::mimeData(const QModelIndexList& indexes) const
 
     if (seenGroups.isEmpty()) {
         delete data;
-        return Q_NULLPTR;
+        return nullptr;
     }
     else {
         data->setData(mimeTypes().first(), encoded);