Ipywidgets-extended

**Fixes**:
- Ensure updating `disabled_options` for an already initialized `DropdownExtended` widget updates it appropriately (5).
The example notebook has been updated accordingly, explaining the resulting selected index in different cases.

**Updates**:
- Ensure expected behaviour if the first option is disabled and if all options are disabled (2).
- Have an example Jupyter notebook to showcase widgets (2).
Currently this only showcases the `DropdownExtended` widget.

**Fixes**:
- Match up metadata for Python part of `DropdownExtended` widget (2).

**Fixes**:
- Properly import version in `dropbox.py`.

**Developer updates**:
- Add module doc-strings.
- Fix publish workflow, avoiding npm distribution files mix up.

First release.

This Jupyter widget package/library will extend the [`ipywidgets`](https://github.com/jupyter-widgets/ipywidgets) widgets with extra options and capabilities.

For now the only extension is to the Dropdown widget, making it able to disable individual options in the list.