Performance improvements for long text edition.

Limit each parse to batches of a few words, to keep the UI thread
responsive.

Possible optimizations for the future:
- SpellCheck in a thread, but that requires some locking mecanism
- Only spell check what is visible on screen. Will require additional
  spans to tag the pieces of text.

Change-Id: Ibf8e98274bda84b7176aac181ff267fc1f1fa4cb