wasm: look into binding QString to javascript

It might be nice to be able to do something like:

QString string =xhr["responseText"].as<QString>();

instead of

QString string = QString::fromStdWString(xhr["responseText"].as<std::wstring>());

or from std::string and skip the conversion.

https://kripken.github.io/emscripten-site/docs/porting/connecting_cpp_and_javascript/embind.html#advanced-class-concepts

Things to consider:

•  memory use

• performance
• do we really need to expose a whole class like QString?
• ?