[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: 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":

   $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:

   $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).