Present consistent listfield display order

Avoid sorting array before returning it.

A couple of small code simplifications.

Cast valueformat to string like other variables are cast.
Show "none" option at top of list.
fixes #808 - ensuring all js ajax called for JFormElements (tables, connections, fields etc) are run through Fabrik.RequestQueue which is checked on element admin page save to ensure that elements are not saved when ajax fields are in the process of updating themselves
Merge branch 'joomla3' of github.com:Fabrik/fabrik into joomla3
Merge pull request #809 from Sophist-UK/fix_listfields

Present consistent listfield display order