OSX: Added auto scroll to WaveFormScrollView; can now change song position using track bar.

- WaveFormScrollView: Added auto scroll for secondary position and an
option in the contextual menu to enable auto scrolling for the current
song position.
- MainWindow: Can now change song position using the song position track
bar (added mouse up/mouse down events to manage this correctly).
- SyncWindow: Fixed bug where the playlist index would be displayed
zero-based.