TextInput: allow input interpretation, user input error reporting and input completion

[deepfire]

• Extend textInput, TextInput and TextInputConfig.
  • Expose the current input position.
  • Give the user control over input event interpretation.
  • Allow the now-exposed input interpreter signal input errors.
  • Provide means for input completion by the now-exposed interpreter.

This is a breaking change wrt TextInput. Suggestions on how to avoid this are welcome!