The new boxes are:
- `this._download_box`: contains all download section items
- `this._upload_box`: contains all upload section items
- `this._sum_box`: contains all sum (download+upload) section items
- `this._metrics_box`: contains all the previous ones

The new tree layout is:

- box
    - metrics_box
        - download_box
            - down
            - downunit
            - downicon
        - upload_box
            - up
            - upunit
            - upicon
        - sumbox
            - sum
            - sumunit
    - icon_box
        - icon

This style should work with the default panel;
it may produce artifacts with shell themes or
extensions which change the panel (e.g. DashToPanel)