Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

Extract common text selection overlay logic from Material to Widget and create a Cupertino version of the overlays

See https://github.com/flutter/flutter/issues/11168

* Provide haptic/acoustic feedback for tap & long-press on Android

* review comments

* fixed example code

* review comments

* comment fix

* Add a text formatter interface used by EditingText. Provide some default implementations.

* self nits

* Handle -1 selection offsets

* review notes

* simplify regular expression

* Add whitelisting formatters. Use a custom phone number formatter in text demo.

* review notes

* not being able to addAll(null) is pretty annoying

* review notes

* partial tests

* Add tests

* didn’t end up needing mockito

* move to services

* make @immutable const

* fix build

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Also, fix an issue where hint text wasn't visible when the
InputDecoration was collapsed.

Fixes #8541

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