Add a #disabled property to form elements

At least once per week (sometimes more often), I answer the question in IRC:

"How do I disable a textfield/checkbox/etc.?"

The answer currently is to do:

$form['#attributes'] = array('disabled' => 'disabled');

However, this is not intuitive; when most people think of #attributes, they think of "extra" things like "class" and stuff... not attributes that are innately part of input, button, etc. elements.

So this patch adds a #disabled property so that you can simply do:

$form['textfield'] = array(
  '#type' => 'textfield',
  '#title' => t("Some textfield you can't change"),
  '#disabled' => TRUE,
);