Sound Effect Support

I've added basic support for playing the following sound effects:
- Command Accepted
- Command Rejected
- Started Listening
- Stopped Listening

I added some UI to the Settings page to choose None, Default, or Sound Pack sound effects. If they select Sound Pack, a folder browser dialog will be displayed. Any missing files will be ignored (ie. if no file is supplied, no sound will be played). The UI could probably be improved, but this is my first crack.

I also included some basic sounds as the Default set which I created with Audacity. We may want to tweak these to be easier to tell apart.

One current issue with the sound playback is that there is no ordering to the playback. For example, the Command Rejected sound might play before or after the Stopped Listening sound. We might be able to add a priority queue or something to add ordering for the sounds.

Another limitation is that the .NET SoundPlayer class I'm using is limited to WAV file playback.