ComboBox: query font & colors via style panel

Advanced custom styles have difficulties binding values from native
styling elements to the style root, because such styling elements are
created inside delegate components.

ComboBoxStyle {
    id: style
    panel: Item {
        NativeStyle {
            id: native
            elementType: "ComboBox"
        }
        // typical "backwards" bindings that we want to avoid:
        Binding { target: style; property: "font"; value: native.font }
        Binding { target: style; property: "foo"; value: native.foo }
        Binding { target: style; property: "bar"; value: native.bar }
        // ...
    }
}

Becomes:

ComboBoxStyle {
    id: style
    panel: Item {
        NativeStyle {
            id: native
            elementType: "ComboBox"
        }
        font: native.font
        foo: native.foo
        bar: native.bar
        // ...
    }
}

Change-Id: I23a3a6528cf0162dc91f94a44bcade2985260a9d
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>