ENH: 1. Deep copy methods on the handles, so that the seed widget can function as a container of handles with heterogenous properties.
    2. When invoking interaction event on the seed widget, also pass the seed index as call data, as a convenience to the user.
    3. Test (2) above.
    4. (BUG) Do not invoke interaction event every time we move the mouse. Only invoke it if we are near a seed.