Create new Key Form API element

[rlhawk]

To make it easy for modules to integrate with Key, the version of the module for Drupal 7 adds a new form API element called key. It extends the select element, but fills #options with the list of available keys. It also provides additional instructions for using and defining keys in the #description. Here's an example of what that looks like in code:

$form['secret_key'] = array(
  '#type' => 'key',
  '#title' => t('Secret key'),
);

This functionality should be in the Drupal 8 version also. Key for D7 has additional functionality, such as filtering, that can be added later for D8.