[meta] Round-up of form buttons, buttons, link buttons, and links

Background

1. #1845728: Refactor and introduce generic button CSS styles for forms and links revamped and generalized the CSS for all possible buttons:

   .button,
button

   Note: Seven does not implement styles for .button.

2. #1238484: [docs update] Ability to mark the form buttons to be of a specific type (so that they can be styled differently) advanced on it and introduced support for specific button "types":

   <?php
$form['actions']['submit']['#button_type'] = 'primary';
$form['actions']['delete']['#button_type'] = 'danger';
?>
   
   ↓
   
   .button--primary { color: blue; }
.button--danger  { color: red; }

   Relevant case study: http://www.lukew.com/ff/entry.asp?571

   #button_style is not limited to 'primary' and 'danger'. The value is simply taken over as .button--$value CSS class name. See http://getbootstrap.com/css/#buttons for common button types.

   Note: Bartik does not implement styles for .button--danger.

3. Based on the last two building blocks, #1719640: Use 'button' element instead of empty links introduced dynamic/on-page buttons for JS interactions which masquerade as links, in order to avoid <a href="#"></a> as well as href="javascript:void(0)":

   <button type="button" class="link">Show summary</button>
   /**
* Show buttons as links.
*/
button.link {
  background: transparent;
  border: 0;
  cursor: pointer;
  margin: 0;
  padding: 0;
}

   Note: Styling is broken in Bartik (faulty font override).

4. #216064: Entity form "Delete" button triggers server-side + HTML5 form validation; change "Delete" button to a link is about to convert all "Delete" buttons into links, so as to avoid HTML5 front-end form validation and needless back-end form processing code:

   <?php
$actions['delete'] = array(
  '#type'=> 'link',
  '#title'=> $this->t('Delete'),
  '#attributes'=> array(
    'class'=> array('button', 'button--danger'),
  ),
);
?>
   
   ↓
   
   <a class="button button--danger" href=".../delete">Delete</a>

   Note: That patch will not adjust/fix the link button styles in Bartik/Seven.

Goal

1. Every button has a .button class or is a <button> or [type=submit] or [type=button].

2. By default, every link in a form actions container has a .button class, so as to appear as a button. A theme is easily able to override this.

3. Every link/button that (1) will trigger a confirmation for a destructive action or (2) immediately causes a destructive action uses .button--danger. A theme is easily able to override this button type styling (in both places).