Reduce number of unneeded copies

This patch aims at reducing the number of copies for obejcts that could
be referenced rather than copied, because they're not modified during
the computation.

src/gui/entry/EntryHistoryModel.cpp

@@ -54,7 +54,7 @@ QVariant EntryHistoryModel::data(const QModelIndex& index, int role) const
 
     if (role == Qt::DisplayRole || role == Qt::UserRole) {
         Entry* entry = entryFromIndex(index);
-        TimeInfo timeInfo = entry->timeInfo();
+        const TimeInfo& timeInfo = entry->timeInfo();
         QDateTime lastModificationLocalTime = timeInfo.lastModificationTime().toLocalTime();
         switch (index.column()) {
         case 0: