Add unique identifiers or classes for further buttons

I just saw that there are already places, where this is done very well, e.g. AddToCartForm.php:

/**
   * {@inheritdoc}
   */
  protected function actions(array $form, FormStateInterface $form_state) {
    $actions['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Add to cart'),
      '#submit' => ['::submitForm'],
      '#attributes' => [
        'class' => ['button--add-to-cart'],
      ],
    ];
    return $actions;
  }

So we just have to find the missing pieces :)

Okay, here's the first shot. For now to show the expectation and to discuss details.

One thing I'm a bit curious about is that typically button--XYZ is a modifier and the button should have "button" as base class? But for example in #2 that's not the case, so I didn't change that yet and used the same naming logic.

This might (already in #2) be against BEM.
In that case, we should either add a "button" class or remove the "button--" prefix?

I'm quite happy to see possible improvements for themers here with minimal effort on programming side.

Let's do this in general and make the commerce world a bit better for themers :)

Why is this categorized as a bug report? This is going to have to wait until we have availability to review this internally...

@jsacksick thanks, my fault. This was initially going into a different direction and then I forgot to change the value. Pardon!