Box Layout: consider spacing and padding when using percentage values for width and height

[Murmele]

• If possible, the change is auto-tested

When using percentage values in the layout, the spacing and the padding are not considered. So with 50% for 2 elements one element goes outside of the layout

export component TestCase inherits Window {
    preferred-width: 300px;
    preferred-height: 300px;

    box:= HorizontalBox {
        rect:= Rectangle {
            width: 50%;
            background: red;
        }

        Rectangle {
            width: 50%;
            background: blue;
        }
    }
}

Old:

New: