Common alias for QMetaType int Qt6

Since Qt6, QMetaType registers custom types under a typename that may vary with respect to the compiler. This is very problematic since the typename was used in dtk as a cross-platform identifier for serialisation and deserialisation processes.

See this forum discussion for an example of the problem that this regression may occur.

dtkCoreParameter makes an intensive use of serialisation/deserialisation process but as this issue may regard any kind of classes, a global solution at the scale of the whole dtk ecosystem has to be featured.

Hence, dtkCoreMetaType now features the singleton dtkCoreMetaTypeRegistry which provides a map between the integer given by qMetaTypeId<T>() and a QString as common custom alias.

In the case of dtkCoreParameter, the MACRO DTK_DEFINE_PARAMETER(type, name_space), which is mandatory to make parameters work, is used to perform the registration into dtkCoreMetaTypeRegistry automatically without any additional code for the developers. The alias will be equal to the first argument of the macro, namely type.

For instance in dtkCoreParameterNumeric.h, one declares an alias for dtkCoreParameterNumeric<double> as follows:

namespace dtk {
    using d_real = dtkCoreParameterNumeric<double>;
}
DTK_DECLARE_PARAMETER(dtk::d_real);

And in dtkCoreParameterNumeric.cpp, the following code is written:

DTK_DEFINE_PARAMETER(dtk::d_real, d_real);

Firstly, this enables to register the type dtkCoreParameterNumeric<double> to the QMetaType system with an additional alias dtk::d_real so that one can instantiate a QMetaType object using this alias:

auto mt = QMetaType::fromName("dtk::d_real");
qDebug() << (qMetaTypeId<dtkCoreParemeterNumeric<double>>() == mt.id()); // must display true

Secondly, it enables to populate the dtkCoreMetaTypeRegistry with the pair {qMetaTypeId<dtkCoreParemeterNumeric<double>>, "dtk::d_real"} so that the serialisation process of the parameter can be done as follows in dtkCoreParameterBase CRTP class:

template <typename Derive>
inline QVariantHash dtkCoreParameterBase<Derive>::toVariantHash(void) const
{
    QVariantHash hash;
    auto alias = dtk::metaTypeRegistry().alias(qMetaTypeId<Derive>());
    hash.insert("type", alias);
    ...
}