Allow fieldsets to be persistent and save focus when AJAXing

Issue #2163197 by Mark Carver: Allow fieldsets to be persistent and save focus when AJAXing.
Committed to master and stage:
https://github.com/dreditor/drupal_org/commit/306579bb2804e70d842e8ed85b...

Example of changes can be seen on both sites:
http://form-drupal.redesign.devdrupal.org/node/2130811 (dev)
http://page-drupal.redesign.devdrupal.org/node/2130811 (stage)

I've rolled back staging to before the patch, so you can see a "before" and "after".

Basically, if you refresh the page or purposefully/accidentally focus or change the project field the entire form is replaced, thus collapsing the fieldset again. It also loses where the focus is (ie: if you tab from the project field, you go to the version version, but it's no longer focused when it's replaced via AJAX).

This patch fixes all that and keeps persistent fieldset collapsed states (leveraging the browser's localStorage) and persistent input focus (on AJAX replacements).

I'll update the patch to be a little more descriptive.

Moving to project_issue after discussing this with @drumm. This is generic code that can be used by all, not drupalorg specific.

Ok, here's a new patch against the project_issue module instead. Interdiff is useless.

Changed some of the namespacing to be consistent.

Example of changes can be seen on:
http://form-drupal.redesign.devdrupal.org/node/2130811 (dev)

Without the changes:
http://page-drupal.redesign.devdrupal.org/node/2130811 (stage)

htaccess: drupal/drupal
login: bacon/bacon

Yes, I think this should be fixed upstream (while putting this in for now, until we can). Honestly though, I think this should really should just be part of core period as part of the FAPI (like https://drupal.org/sandbox/arshad/1202674 does), it's relatively small code. The reason I didn't use that sandbox was mostly for time. I would have to rewrite a lot of it to not use DB variables. Using the DB for storage seems a completely unnecessary waste and ultimately a high cost considering issues are probably the most active part of d.o. It could also be somewhat unpleasant if we had to do AJAX calls each time the state was retrieved or saved, not really my cup-o'-tea.

The thing I really liked about it though is the #persistent value, I could incorporate something similar and ad a data-collapsible="true" to the fieldset tag:

$form['block'] = array(
  '#type' => 'fieldset',
  '#title' => t('Block configuration'),
  '#weight' => 3,
  '#collapsible' => TRUE,
  '#collapsed' => FALSE,
  '#persistent' => TRUE,
  '#tree' => TRUE,
);

Then we could specifically target only the fieldsets we want (ie: metadata, files).

Another thing I ran into is that, technically, the form is same for both the view and edit aspects of the node. If one expands or collapses the fieldset say on the node view the same will be applied if they go to the node edit. I think maybe we should add a new key value: #persistent_prefix which, by default, would use the form_id. This way we could amend the form_id with _embeded for use '#persistent_prefix' => $form['#form_id'] . '_embeded' for the node view.

Thoughts?