Improved the smiley image TextButton example a little. Handling the case where the `Future.delayed` object that represents the button's one second long action is superseded by a second button press that triggers a new one-second action. This does complicate the example - just a little - but it's a little more robust. In case someone copy-and-pastes the code.

The TextButton example was recently updated: https://github.com/flutter/flutter/pull/144583