child property as second parameter

[Aylur]

Example Media widget currently looks like this

function Media() {
    // declare components
    // and then return the layout
    return Widget.Box({
        class_name: 'player',
        children: [
            img,
            Widget.Box({
                vertical: true,
                hexpand: true,
                children: [
                    Widget.Box({
                        children: [
                            title,
                            icon,
                        ],
                    }),
                    artist,
                    Widget.Box({ vexpand: true }),
                    positionSlider,
                    Widget.CenterBox({
                        start_widget: positionLabel,
                        center_widget: Widget.Box({
                            children: [
                                prev,
                                playPause,
                                next,
                            ],
                        }),
                        end_widget: lengthLabel,
                    }),
                ],
            }),
        ],
    });
}

With this PR its possible to save some indents, and less child and children keyword

function Media() {
    return Widget.Box(
        { class_name: 'player' },
        img,
        Widget.Box(
            {
                vertical: true,
                hexpand: true,
            },
            Widget.Box({},
                title,
                icon,
            ),
            artist,
            Widget.Box({ vexpand: true }),
            positionSlider,
            Widget.CenterBox({
                start_widget: positionLabel,
                center_widget: Widget.Box({},
                    prev,
                    playPause,
                    next,
                ),
                end_widget: lengthLabel,
            }),
        ),
    );
}

two issues that arise with this

1. the object parameter can feel wrong

Widget.Box(
    { // this gets pushed one indent deeper
        vertical: true,
        hexpand: true,
    },
    Widget.Box({},
        title,
        icon,
    ),
)

this could be mitigated with custom formatters

2. the object parameter will have to be passed even when empty

Widget.Box({},
    title,
    icon,
)

this can be solved somewhat redundantly, but honestly I don't think we need to, we can just assume there will always be an object passed, only time I can imagine this happen is with Box, which should instead also take an array of widgets as its first parameter, just like how Label can take a string, but thats going to be another commit

edit: both are valid

Widget.Box({},
    title,
    icon,
)

Widget.Box([
    title,
    icon,
])

3. Widgets not fully subclassed, but only with Widget.subclass utility, will be typed as if they can take a child
3. Widgets not fully subclassed can't take a child as second parameter for simplicity