Add please Qt6 support

[NaGeMaGe]

[juangburgos]

I haven't even downloaded Qt6. Updating my projects to Qt6 seems far off, given that their new open-source-unfriendly attitude seems to lock LTS versions only to paying customers.

Having said that, why not try to compile it as it is? And if you run into any trouble, I can offer advise on how to fix things.

[NaGeMaGe]

QDeferred/qdeferreddata.hpp:195: error: ‘Recursive’ is not a member of ‘QMutex’
In file included from QDeferred/qdeferreddata.cpp:1:
QDeferred/qdeferreddata.hpp: In constructor ‘QDeferredData::QDeferredData()’:
QDeferred/qdeferreddata.hpp:195:18: error: ‘Recursive’ is not a member of ‘QMutex’
195 | m_mutex(QMutex::Recursive),
| ^~~~~~~~~

[NaGeMaGe]

I replaced QMutex with QRecursiveMutex and the library was compiled. But I'm not sure if everything is working as expected.

[juangburgos]

Run Test01, that cover most of the API.