New animations and other improvements to single-line text field

Error message is set from .kv language (eg "error_message: err123")

Error message can be changed using "self.root.ids.text_field.error_message = " or similar

Error message now fades instead of popping in and out

Bottom line now retracts on loose focus, instead of vanishing

Hint message will fade to error colour (if error)

Currently hint fades back to grey and remains at the top after loosing focus if there is text, but that can be changed to fade away if you prefer.

Also a few PEP8 improvements to kitchen_sink

If you haven't noticed already, I like fade effects 😄