Qt6 property bindable support

[arBmind]

Note this is a breaking change to the way Verdigris works internally.

Sorry that a lot of changes accumulated here. My intention was originally to mainly speed up compilation times. So I simplified the code base to only C++20. Later I found this to be the base to implement bindable. Here goes nothing…

New requirements:

• Qt6.2.4+ (Versions before are too buggy with C++20 support, which is still an ongoing issue.)
• Compiler with decent C++20 support (CLANG >=12, GCC >= 12, MSVC 2022 >=17.2, MSVC 2019 >=16.11)

New features:

• Use current property flags from Qt 6.3
• Also support the REQUIRED property
• Reduced the global namespace spam (macros are bad enough)
• W_PROPERTY now supports bindable with overload support (mainly for: const / non-const overloads)
• Added tst_qproperty.cpp from Qt6 to test bindable support
• Allow defining overloaded methods in macros with W_SLOT_N, W_INVOKABLE_N, W_SIGNAL_N or W_SIGNAL_COMPAT_N.

Faster compile times:

• use C++20 concepts for complex reflections where possible
• use pointers instead of inheritance for counters and strong flags
• dropped compile time hash for MSVC with wrapped member function pointers

This enables some cleanups:

• Dropped Qt5 based MOC versions
• Removed Qt5 unit tests
• Cleaned up C++ version precompiler checks all over the code
• Dropped old compiler unit tests
• Require semicolon after the declaring W_ macros to avoid tooling confusion (clang_format, QtCreator)

While I was on breaking changes, I also upped the project a bit:

• Introduced clang-format to reduce manual formatting work
• Dropped AppVeyor that was only used for old msvc tests

If we want to move forward with this, we should do:

• [ ] Gather support for this PR. Or propose changes.
• [ ] Drop Appveyor from the CI list.
• [ ] Archive the old master branch.
• [ ] Update the readme.

[arBmind]

Fixes #84