Add support for clearForm and resetForm methods via FAPI

The form plugin contains several useful methods that developers or site admins may wish to attach to forms.

Two of these are clearForm and resetForm (mainly the former, since the latter is generally natively supported by browsers) without JS).

clearForm clears all values entered into a form. It is invoked by the plugin's ajax methods but may also be useful directly, to allow users to clear values from a form when e.g. they change their minds.

The attached patch adds support for these two methods via the Forms API.

We begin by registering a new element type, 'push_button'. This is needed because the 'button' form element type is hard-coded (see theme_button() to be of type "submit", whereas here we'll want "button" and "reset" input elements.

For the clear button, we use CSS to hide the element is Javascript is not present. We use a #process callback to add the JS and CSS as needed and to add classes that will trigger the behavior. Finally, in a JS file, we process elements and attach the behaviour.

To test, add the following function to the jquery_form.module file:

function jquery_form_form_alter($form_id, &$form) {
  if (!isset($form['clear_button'])) {
    $form['clear_button'] = array(
      '#type' => 'push_button',
      '#value' => t('Clear'),
      '#button_type' => 'clear',
    );
    $form['reset_button'] = array(
      '#type' => 'push_button',
      '#value' => t('Reset'),
      '#button_type' => 'reset',
    );
  }
}

This will add a reset and a clear button to every form.

We could use the same general approach to enable adding of AJAX behaviours to forms via Forms API or via admin UI. I'll open a separate issue with suggestions about that.