Introduce undefined values for properties and struct fields about slint

[ogoffart]

We could have default value for struct like propose in #2936 , you could do something like:

export struct Style {
    border-radius: length,
    border-top-left-radius: length = -1px,
    background: brush
}

component StyledRectangle {
      in property <Style> style;
      
      Rectangle {
            background: style.background;
            border-radius: style.border-radius;
            border-top-left-radius: style.border-top-left-radius >= 0 ? style.border-top-left-radius : style.border-radius;
      }
}

export component MyExample inherits Window {
      width: 600px;
      height: 400px;
      
      StyledRectangle {
            style: {
                  background: #000000;
                  border-top-left-radius: 8px;
            }
      }
}

But in general, we might want to have option<...> types.

[FloVanGH]

border-top-left-radius: style.border-top-left-radius >= 0 ? style.border-top-left-radius : style.border-radius;

I think from user perspective that looks more like a workaround. As user I want to do something like on the Rectangle directly, if I do not define a value for border-radius it should be ignored and it should not override values like border-top-left-radius.

[ogoffart]

Right, for the general case, we would need to introduced a new type like option<length> or length? or something like that.
Somehow related: #615