Add QDir::copy

Consider adding a convenience function to copy local directories recursively.

This is a common operation that currently has to be reimplemented anew, and we have multiple instances of it in qtbase alone. It seems also a commonly searched for feature, see e.g. https://stackoverflow.com/questions/2536524/copy-directory-using-qt

Alternative is to rely on std::filesystem::copy. Anyhow, this fails for qrc resources (and extracting qrc resources to a local directory is actually a common use case).