Raising Qt requirement to Qt 5.4

[Kai Uwe Broulik]

> > The important new features in Qt 5.4 in my opinion for frameworks development 
> > are:
> > * QSignalSpy(const QObject *object, PointerToMemberFunction signal)
> > * Q_LOGGING_CATEGORY(name, string, msgType)
> 
> + QTimer::singleShot(timeout, receiver, PointerToMemberFunction)
> + QVector::removeOne
> + QQuickItem::grabToImage, used by KDeclarative

+ QSGSimpleTextureNode::setOwnsTexture

Which will allow us to remove our own ManagedTextureNode class