Method onResize

tests/Demo.cpp:228–239

    }

    void onResize(const ResizeEvent& ev) override
    {
        const uint width  = ev.size.getWidth();
        const uint height = ev.size.getHeight();
        const double scaleFactor = getWindow().getScaleFactor();

        bgIcon.setWidth(width - 4 * scaleFactor);
        bgIcon.setHeight(width - 4 * scaleFactor);

        lineSep.setStartPos(width, 0);
        lineSep.setEndPos(width, height);
    }

private:
    Callback* const callback;