Add support for C++ QML Elements in CxxQtBuilder

[akiselev]

This PR is a WIP to add support for registering classes implemented in C++ with a QML module so that they can be included in Cargo-only builds. Instead of just running moc on a header, it also includes the generated metatype information which is needed to register the C++ QML elements.

This first implementation is one I wrote to unblock my own project so I'd appreciate some advice for how to integrate this properly into cxx-qt-build. It should probably be another property on QmlModule like so, but I couldn't figure out how to best implement it that way:

    let module: QmlModule<&str, &str> = QmlModule {
        uri: "Clippo.Theme",
        rust_files: &[
            "src/theme.rs",
        ],
        cpp_files: &[
            ("src/ThemeManager.h", "src/ThemeManager.cpp"),
        ],
        ..Default::default()
    };

With @LeonMatthesKDAB's temporary fix found in #1135, it correctly includes C++ QML elements in modules built with Cargo only.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (f5afefe) to head (82efc03).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           71        71           
  Lines        11967     11967           
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  Hits         11967     11967           

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[redstrate]

I think this deserves a mention in CHANGELOG.md, can you add it to the MR?

[redstrate]

cpp_qobject ? I guess this could have other applications outside of QML

[LeonMatthesKDAB]

Hi @akiselev ,
as you mentioned, the best way to integrate this is indeed as a property on QmlModule.
The design of the QmlModule type is unfortunately a bit overly complicated, as it uses two template types, which we really don't want to expand.
My suggestion is to just hijack the existing A type, like we do for qrc_files.

I'm okay with the "list of tuples" approach you have in your initial comment.
We could even consider just a plain list of cpp_files, as we should be able to differentiate between .cpp and .h files by the extension.

Please note that for version 0.8 we are planning a larger refactor of the build system, which will likely end up supporting what you're currently asking anyway due to the nature of the proposed architecture.
See also: #1125
(basically: The qobject_header call should in future take care of registering automatically, if you're buidling a Qml module).