Allows Checkbox List and Dropdown List Property Editors to be sortable (U4-3951)

This pull request contains an amend to allow the user to amend the sort order of the prevalues for a checkbox or drop down list.  Previously you could only add and remove them.  

The UI amend is straightforward.  What was a bit more tricky was an issue discovered where the order of the prevalues was lost when retrieved from the API (as it came back as a JSON object rather than an array).  I got around this by ensuring the sort order is returned along with the value, allowing the order to be restored in JavaScript.

See discussion on the issue tracker for more detail on this: http://issues.umbraco.org/issue/U4-3951