Improvements to the QML type system

Currently, QML's type system is not expressive enough to express quite a few things. Instead, we have to fall back to var properties.

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This should be improved to help tooling (LSP + compiler), to ease documenting what can be provided, and to allow developers to express the correct type of their properties.

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See also https://wiki.qt.io/Improving/extending_QML_type_system