WIP: Don't skip Option for nullable collections

[fengalin]

Nullable collection were represented as a Vec<_> or [_], but an empty collection is not the same as an undefined collection.

Fixes: #1133
See also: gtk-rs/gtk-rs-core#1257

TODO

• Add support for glib::value Option<Vec<_>> conversions.

[sdroege]

[ ] Add support for glib::value Option<Vec<_>> conversions.

I think this also involves untangling our existing implementations a bit as they currently map NULL to empty arrays, and possibly also the other way around?

[sdroege]

This looks too easy, I didn't expect that :)

[sdroege]

There's also #1551 doing more or less the same (but a bit more?). I think the main problem here now is fixing glib/translate.rs

[sdroege]

@fengalin What's the plan to continue here, especially with the translation traits?

[fengalin]

My plan was to start over from the PR you linked and implement the changes in the glib bindings. But I got involved with other stuffs. I'll try to resume this later this week.