add SampleMusicPreviewMode_LastSong, now that it works.

There is a hack in there for playing the song associated with the Random option, based on Takuya's DDR Extreme theme having music for it. I am not sure if this is accurate behavior, though.

src/GameConstantsAndTypes.cpp

@@ -418,7 +418,7 @@ static const char *SampleMusicPreviewModeNames[] = {
 	"Normal",
 	//"StartToPreview",
 	"ScreenMusic",
-	//"LastSong"
+	"LastSong"
 };
 XToString( SampleMusicPreviewMode );
 StringToX( SampleMusicPreviewMode );