Add replacements for QML_ELEMENT macro and friends

[arBmind]

Qt5.15 introduced new macros to register QObject with the Qml type system.
See "QtQml/qqml.h" header:

• QML_ELEMENT
• QML_SINGLETON
• QML_NAMED_ELEMENT(name)

All these macros use Q_CLASSINFO internally. This can also be done with Verdigris.

Ideas and help is more than welcome!

[arBmind]

My current state of analysis is:

• The QMetaObject part is quite easy. (I can do that part in an hour or two)
• But we need some build system support to generate the necessary *_qmltyperegistrations.cpp and *.qmltypes. Qt deploys an army of CMake scripts to invoke moc multiple times to get all these. So far we do not even have CMake build support in the Verdigris project. (QMake cannot do that. I doubt it will be easy in Qbs.)

My sketchy plan:

1. Add CMakeLists.txt to Verdigris.
2. Create a Verdigris replacement for qt_add_qml_module.
   • This will require a deep knowledge of CMake and some inspiration from the cmake tools provided by Qt.
3. Use compiler defines to build an qmltyperegistrations.cpp equivalent.
   • Option A: build an executable that when executed generates the .cpp file.
   • Option B: generate the #include statements with CMake and use some local type registration to effectively do the same.
   • Option C: generate the .json compatible to moc and use qmltyperegistrar to build the qmltyperegistrations.cpp.
4. Once this works use some clever tricks to build an executable that generates the qmltypes.

Seems kinda impossible.