NEW: Setters and getters for constructor arguments

Based on help from Willr (http://www.silverstripe.org/customising-the-cms/show/17426) I've created setters and getters for the constructor of the AjaxUnifiedTextField. No API change. For a future version though we can cleanup the constructor to be identifical to Textfield without losing functionality.

I notice that the order of arguments is also currently not consistent with TextField at the moment.

It's possible to add deprecation warnings to the constructur use of these arguments, although I haven't done so.