After this patch, there are three major text input widgets:
* EditableText. This widget is a low-level editing control that
interacts with the IME and displays a blinking cursor.
* TextField. This widget is a Material Design text field, with all the
bells and whistles. It is highly configurable and can be reduced down
to a fairly simple control by setting its `decoration` property to
null.
* TextFormField. This widget is a FormField that wraps a TextField.
This patch also replaces the InputValue data model for these widgets
with a Listenable TextEditingController, which is much more flexible.
Fixes#7031