Consistent AHAH throbber, ahah.js bug fixes

This patch adds the correct support for updating the progress bar indicator. I'll follow up with code examples.

@webernet You're correct, I don't think a width is wanted on all progress bars at all, and 5em is too narrow anyway for places that do use ahah.js (such as upload module). I've changed the width to 16em and only on .ahah-progress-bar class, which should allow space for a short message and percentage (if needed). The width is needed at some point though, because it keeps the appearance correct when floating it left.

The progress bar indicator is the main component of this patch. Since modules utilizing AHAH will need different approaches, we need to accommodate for all scenarios. Sample displays attached. Here are some code examples of how you would customize the progress bar used with AHAH to your liking:

Example 1. Throbber (default).

  $form['element']['#ahah'] = array(
    'path' => 'some/path/js',
    'wrapper' => 'wrapper-id',
    'progress' => 'throbber', // Optional line, throbber is default.
  ),

Example 2. Throbber with message.

  $form['element']['#ahah'] = array(
    'path' => 'some/path/js',
    'wrapper' => 'wrapper-id',
    'progress' => array('type' => 'throbber', 'message' => t('Please wait...')), // Value is an array if multiple settings are set.
  ),

Example 3. Progress bar.

  $form['element']['#ahah'] = array(
    'path' => 'some/path/js',
    'wrapper' => 'wrapper-id',
    'progress' => 'bar', // A simple bar.
  ),

Example 4. Progress bar with message.

  $form['element']['#ahah'] = array(
    'path' => 'some/path/js',
    'wrapper' => 'wrapper-id',
    'progress' => array('type' => 'bar', 'message' => t('Please wait...')), // Bar with message (upload module, book), gets the t() string out of javascript and makes it customizable.
  ),

Example 5. Progress bar with message and progress indicator.

  $form['element']['#ahah'] = array(
    'path' => 'some/path/js',
    'wrapper' => 'wrapper-id',
    'progress' => array('type' => 'bar', 'message' => t('Please wait...'), 'path' => 'some/path/progress')), // Potentially great for expensive or time-consuming tasks.
  ),

Example 6. No progress indicator at all.

  $form['element']['#ahah'] = array(
    'path' => 'some/path/js',
    'wrapper' => 'wrapper-id',
    'progress' => 'none', // No throbber or bar. Adds nothing to the page, but still disables the changed element during request.
  ),

webernet and I had a discussion in #drupal about the class names used, reporting inconsistency between the progress class and the ahah-progress and ahah-progress-throbber class. Here are the classes used an my reasons for using them:

The throbber version:

<div class="ahah-progress ahah-progress-throbber">
  <div class="throbber">&nbsp;</div>
  <div class="message">Please wait...</div>
</div>

The progress bar version:

<div id="ahah-progress-element-id-here" class="progress ahah-progress ahah-progress-bar">
  <div class="bar">&nbsp;</div>
  <div class="percentage">&nbsp;</div>
  <div class="message">Please wait...</div>
</div>

So basically the classing here let's us easily select the following items:
- All progress elements added by ahah.js (.ahah-progress)
- Specifically ahah progress bars (.ahah-progress-bar)
- Specifically ahah progress throbbers (.ahah-progress-throbber)

The class .progress is not used in the throbber version of the code because it's probably best if that class is reserved for use exclusively by progress.js. So after reviewing my code a second time, I don't think that the class names or CSS should be changed. Not to say it's perfect. Suggestions are very welcome!