SingleLineTextField: Add "required" parameter. Will throw an error when there is no text

Makes it easier to do this: video, in that you no longer have to edit the module to get this to work.

You can now set the textfield's required variable to True, and it will do all of the error handling for you. Will error after the user has opened the textfield, and typed nothing and then closed, or if the user types something and then takes it all away. It can also update its error status while the user types, so they can know before they close the textfield.