[radio-button] Radio group should be vertical by default

[jouni]

The usability of a radio group is much better when the options are laid out vertically. The text is easier to skim and it is much easier to associate the correct button with its label.

We should move the horizontal option into a theme variant instead.

[web-padawan]

This is implemented in vaadin/vaadin-radio-button#133 (also, changed theme to orientation).

[web-padawan]

Reopening as we will revert LitElement based implementation for now in vaadin/vaadin-radio-button#187, will re-implement this later.

[web-padawan]

This is covered by the new base styles. Closing as completed.