Fix media manager upload and subfolder creation

This is based on a comment from @illovo on #3551 and the forum post linked by him (it helps that I speak german *g*).
To me, it looks like indeed the correct fix to the issue.

* The JS code was trying to use the index instead of the actual value of the array and thus failed to update the hidden form fields.
* The readonly path field in the subfolder creation part was missing the `update-folder` class and thus the JS code didn't update it with the new path.

### Testing
* Try creating a subfolder in the media manager
* Try uploading a file to a subfolder in the media manager

#### Tracker
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33413