3.0 formhelper datetime

Part of #2267. This is the first of several changes to update the datetime input generation to use the widget classes. I have tried to preserve most if the option names from 2.x, as it was simple to do. I have removed the dateFormat and timeFormat parameters as they are now options/templates. The separatorshave also been removed from the default templates, as that content is easy to generate with css/templates now.