[#33779] JLayout and Event cleanup of categories views

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33779&start=0

This gets the categories_default_items JLayout to actually output data

It also adds in a more standardized set of events and implements this into the JViewCategories class
In case you use an array here you just need to do an:

```php
extract($displayData);
```

And all the array variables will be available. 

Also it would be interesting to document the variables available on the layout. Something like you did here:
https://github.com/joomla/joomla-cms/blob/staging/layouts/joomla/form/renderfield.php#L12-L18