Add support for bindable properties (Qt6)

[arBmind]

Qt6 introduced bindable properties that replaces manual notify signals.
See Qt documentation:

• https://doc.qt.io/qt-6/qobjectbindableproperty.html
• https://doc.qt.io/qt-6/qobject.html#Q_PROPERTY

For now this is not supported by Verdigris.
Is this even possible?

Ideas and pull requests are welcome!

[arBmind]

My branch: https://github.com/arbmind/verdigris/tree/feature/qt6_bindable implements full support for BINDABLE in Qt 6.2.4+ with C++20.

So far this does not align with the Verdigris project. That works on old Qt 5.x version and requires only C++14 to work.
I have no spare time right now to backport my changes to these requirements.