Added FFMPEG as default coverter, and bug fixes

* FFMPEG is now used by default to convert tracks (You can revert to
NAudio in the settings). This should fix numerous issues.
* Bug fixes.

Clean up

Changes:

* Moved "Hold to play" to settings.
* Fixed cancelation of BackgroundWorker on FormClosing event.
* Changed icon.

v1.3.1

* Fixed System.Runtime.Serialization.SerializationException exception.