Don't implicitly cast iterator to const_iterator

It prevents detachments:
To illustrate:

QMap<QString, QString> map;
/* code compiles and works fine but find() returns the non-const
   QMap::iterator that detaches!
*/
QMap<QString, QString>::const_iterator it = map.find("girish");

but also some subtle bugs:

QHash<int, int> wrong;
if (wrong.find(1) == wrong.cend()) {
    qDebug() << "Not found";
} else {
    /* find() detached the container before cend() was called, so it
       prints "Found"
    */
    qDebug() << "Found";
}

QHash<int, int> right;
if (right.constFind(1) == right.cend()) {
    qDebug() << "Not found"; // This is correct now !
} else {
    qDebug() << "Found";
}

Enforced by QT_STRICT_ITERATORS definition.

src/gui/torrentcontenttreeview.cpp

@@ -66,7 +66,7 @@ void TorrentContentTreeView::keyPressEvent(QKeyEvent *event)
     Qt::CheckState state = (static_cast<Qt::CheckState>(value.toInt()) == Qt::Checked
                             ? Qt::Unchecked : Qt::Checked);
 
-    QModelIndexList selection = selectionModel()->selectedRows(TorrentContentModelItem::COL_NAME);
+    const QModelIndexList selection = selectionModel()->selectedRows(TorrentContentModelItem::COL_NAME);
 
     for (QModelIndexList::const_iterator i = selection.begin(); i != selection.end(); ++i) {
         QModelIndex index = *i;