Fix callback userdata mutability

In some places, building userdata for FnMut callbacks consisted in taking an immutable reference to the user provided function, before converting it to a mutable pointer. This was UB and could lead to future optimizations considering a closure's captured context is immutable.

See also:

• gir update: https://github.com/gtk-rs/gir/pull/1614
• gtk-rs-core update: https://github.com/gtk-rs/gtk-rs-core/pull/1566

TODO:

• update gir submodule & regenerate when matching PR is merged.