Temporarily use onKeyUp event in the ToDoApp example to make it work with Firefox

Use `onKeyUp` temporarily because `onKeyPress` does not work on Firefox with current version of react
See https://github.com/facebook/react/pull/1956

It actually also fix the example with Chromium because there also seems to be problems with `onKeyPress` & `Enter` & `e.keyCode` in react - when I tested `e.keyCode` was always 0 (there is no such problem with `onKeyUp`) so using `e.key` instead with `org.scalajs.dom.extensions.KeyValue` when it's available might be a good idea.
See https://github.com/scala-js/scala-js-dom/blob/master/src/main/scala/org/scalajs/dom/extensions/KeyValue.scala
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