Implement roving tabindex mixin

[web-padawan]

There is a part of duplicating logic in different components:

• vaadin-accordion 
• vadin-list-mixin

This logic should be extracted to a separate mixin so we can reuse it.

We might include _vertical as a protected property, and decouple it from orientation.
So technically it would allow us to have vertical accordion, alongside tabs / list box.