Reference Examples: Avoid static_cast on function arguments

qobject_cast is safer and not very expensive here.

Change-Id: Ie87219e2a5092e453d257064a95a790de31015f5
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

examples/qml/referenceexamples/extended/lineedit.cpp

@@ -51,7 +51,7 @@
 #include <qqml.h>
 
 LineEditExtension::LineEditExtension(QObject *object)
-: QObject(object), m_lineedit(static_cast<QLineEdit *>(object))
+: QObject(object), m_lineedit(qobject_cast<QLineEdit *>(object))
 {
 }