* Allow a widget to be specified for the textfield count, and allow no count at all

* Test all possible states for counter and counterText

* Docs for counter

* counter is a function that generates a widget

* Tests use counter as function

* Fix analyze error in docs

* InputDecoration has counter widget, TextField has buildCounter function

* InputDecorator tests expect counter to be widget again and include
buildCounter

* counter widget example that might actually fit

* Clarify accessiblity concerns in docs

* Include isFocused param for accessibility

* Fix analyze error

* Improve docs per code review

* Rearrange getEffectiveDecoration a bit for clarity

* Fix analyze error about hashValues params

* Clean up docs and redundant code per code review

* Code review doc improvement

* Automatically wrap buildCounter widget in a Semantics widget for accessibility