Create a more generic ordering form field

Replaces 'custom' ones in com_contact, com_banners, com_weblinks, com_newsfeeds and com_plugins.

Weblinks and Plugins actually extend the generic form field. The rest just straight use the generic field.

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31930&start=0