Issue #84 - Implement language dropdown component

Following up with the recent introduction of Custom Elements, this component accepts an arbitrary amount if items and dispatches change event when an item is clicked.

Attributes

There are at least 4 attributes accepted by this component:

• disabled to avoid it working at all
• expanded to check/set expanded state
• items assigned directly or as template literal attribute item=${[one, two, ...]} to populate the least
• placeholder to specify which text should be shown when the list box is visible/opened

All accessibility tests/attributes are in place too.

If you clone this project locally remember to npm run test:io-list-box.js and then npm run start to check in the smoke tests folder the io-list-box.html page.

Ready for review.