Mac: Fixed bug + huge code cleanup in SongGridViewControl.

- SongGridViewControl: Huge code cleanup for the Render method. Split
the method into several smaller methods (DrawRows, DrawCell,
DrawAlbumArtZone, DrawHeader, DrawDebugInformation). Of course this
makes a huge difference in maintenability, thanks to Uncle Bob.
- SongGridViewControl: Fixed bug where the whole background would not be
refreshed when importing new audio files (with a count that was lower
than the previous album and would not fit all screen with rows).