[FEATURE REQUEST] Improve usability of `.css_classes` in builders

[hwittenborn]

Currently in builders (such as LabelBuilder), css_classes requires a Vec<String>, which is quite annoying to use, with snippets such as these littered throughout my codebase (this isn't an actual example, but it shows the point):

...
.css_classes(vec!["background".to_string(), "error".to_string()])

The fact that I have to run .to_string() really makes it messier. It would be nicer if I could just pass in &str references so I could do this:

...
.css_classes(vec!["background", "error")])

It probably wouldn't be best to just break things though - maybe a Vec<impl ToString> or Vec<impl AsRef<str>> could be passed instead? I just don't really want to have to be typing .to_string() everywhere, and I don't really see any reason why it'd have to stay as such.

[hwittenborn]

On that note, it might be nice to even just accept &[impl AsRef<str>] or something like that so it's not forced to be a vector either. I think it'd be acceptable to convert such into a vector inside of the css_classes function internally if needed.

[bilelmoussaoui]

See gtk-rs/gir#1122

[bilelmoussaoui]

Also please check #1110

[hwittenborn]

Is this just waiting for someone to submit a PR then @bilelmoussaoui? I'd be more than glad to look into it if so

[bilelmoussaoui]

I would wait until gtk-rs/gir#1417 lands first