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/src.pro

@@ -57,6 +57,7 @@ include(../version.pri)
 DEFINES += QT_NO_CAST_TO_ASCII
 # Efficient construction for QString & QByteArray (Qt >= 4.8)
 DEFINES += QT_USE_QSTRINGBUILDER
+DEFINES += QT_STRICT_ITERATORS
 
 INCLUDEPATH += $$PWD