Make the following updates to the TextInput API, to improve performance and to align API with that of new TextArea:
- Add a getCharacters() method that returns a CharSequence representing the text input's content.
- Rename insert()/delete() to insertText()/removeText(), and change insert behavior such that the caller is responsible for performing the delete rather than the insertion method.
- Rename TextInputTextListener to TextInputContentListener.
- Change charactersInserted() to textInserted() and charactersRemoved() to textRemoved(); replace characters argument in textRemoved() with a character count to optimize performance (eliminate unnecessary string copy).
- Make text insert/remove events vetoable; re-implement "strictValidation" using these events.
- Remove getTextLength(); superseded by getCharacters().length().