Allow specifying a default via Default in a field within Properties

[carlosmn]

Given a struct like

#[derive(Copy, Default, Clone, Debug, PartialEq, Eq, glib::Enum)]
#[enum_type(name = "SimpleEnum")]
pub enum SimpleEnum {
    #[default]
    One,
    Two,
}

you currently have to specify the default value as part of the param spec builder, e.g.

            #[property(get, set, builder(SimpleEnum::One))]
            fenum: RefCell<SimpleEnum>,

With this PR, we allow for that to change to

            #[property(get, set, default)]
            fenum: RefCell<SimpleEnum>,

as discussed in #930. This started with what @ranfdev wrote though it changed a little bit.

It's still a bit messier than I was hoping as you still have a few different ways of setting the default value, you can have

#[property(default)]
#[property(builder(Type::Default))]

which is fine for compat and because not every enum has a built-in default, but you could also do things like

#[property(builder(Type::Default).default_value(Type::OtherDefault))]
#[property(default, builder().default_value(Type::OtherDefault))]

though you shouldn't and probably wouldn't because why would you. It comes out of the generic nature of this and it's probably not worth catching this and erroring out, but it doesn't make the whole thing easier to understand.

The main thing here are Enums but I also added it for chars because they do have a default value in Rust. This might however not make as much sense in glib? I'm not sure if there's an actually-correct answer there.

As I mentioned in #930 (comment) I don't think you can make this an actual blanket implementation because you need to know a lot about the types and the builder function, which is arbitrary. But covering enums sounds like it's 90% of the use-case, and other libraries can still implement it themselves.

[sdroege]

I think this generally makes sense. thanks! @bilelmoussaoui ?

[bilelmoussaoui]

I think this generally makes sense. thanks! @bilelmoussaoui ?

lgtm to me as well!