alert-dialog: Allow for None parent

Guido Güntherrequested to merge
guidog/libadwaita-rs:alert-dialog-choose-none-parent into master

This makes it match the C API

I cargo culted the bits for the future from other, autogenerated ones. Hope that's o.k. too, otherwise happy to learn how to do it correctly.

Closes: #38 (closed) Signed-off-by: Guido Günther agx@sigxcpu.org

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