FLAC library switch and minor improvements

Switched from jFLAC to a fork of jFLAC called JustFLAC. It offers bug
fixes and support for more FLAC formats.

Popup is now displayed if duplicate track numbers are detected.
Streamline uses the track numbers for temporary file names. If they are
not different, one track may get overwritten by another track during
the conversion process.

Added isolateTrackNumber() to extract the track number from the track
number tag.
Example Cases:
Before Format: 1.
After Format: 1

Before Format: 01/05
After Format: 01