Show fieldset descriptions in modules, menus, plugins parameters

Same as PR #2686, but against staging.

### Issue
Fieldset descriptions don't show up. This is because the code is currently in the layout `joomla.edit.fieldset` but we don't have this description information there. It looks for a property of the (assumed) object `$fieldSet` while in fact `$fieldSet` is an array of fields.
This is the case everywhere except for the component parameters itself. So in modules, plugins, menus and templates parameters and maybe also other places.

### Solution
Moving the code to the `joomla.edit.params` layout where this information (the object) is present.

### Testing Instructions
Try to add a description to a fieldset. For example in modules/mod_articles_news/mod_articles_news.xml change 
```
<fieldset
	name="advanced">
```
to 
```
<fieldset
	name="advanced" description="test">
```
Currently, the description isn't shown, after the patch is applied it will show.

### Known Limitations
This patch only covers "custom" fieldsets. The basic ones (like `basic`, `description`, `request`) are threated differently in the code and thus need to be taken care seperately.

#### Tracker
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32958