[C150_I] Continued cleanup and organization of Phrase class

- Added GetNextWords method, also sets IsDeadEnd for class. Not the best place,
  but the easiest for now.
- GetNextPhrase and GetNextSignificantPhrase now use GetNextWords method to
  get the list of words. This is the method based solution in place of using
  delegates to clean up the code. It "smells" better too, I think.
- Even though the SubPhrases and SubPartialPhrases aren't used by the solution,
  I've left them in for easy exploration of the non-significant subphrases.