Don't detach QVariant in setProperty()/QMP::write()

QMetaProperty::write (and by extension, QObject::setProperty) perform a detach of the passed in QVariant value. The argument to QMetaProperty::write is a const QVariant&, and the documentation gives no warnings of anything funny going on.  The setter of the property we're trying to write takes a const CustomType& so there is no reason to suspect a detach of the QVariant is going to happen. 

But, in QMetaProperty::write, first a copy of the value is taken. This is still shallow. But a little bit later, where the value is stuffed into a Q_ARG in order to call the setter via QMetaCall, QVariant::data is called on the non-const copy of the value. And as ::data isn't const, we get a detach and thus a deep copy of the value there.

As a user, I did not expect this API to cause a detach. There is no warning about it in the documentation, and it takes a const QVariant&, and the setter is also using const ref. Perhaps constData() could be used instead of data()?