MediaFile: add track number to toString (if available).

core/src/main/java/com/jdbernard/wdiwtlt/db/models/MediaFile.java

@@ -18,5 +18,7 @@ public class MediaFile extends Model {
     public Timestamp lastPlayed;
     public String comment;
 
-    public String toString() { return name; }
+    public String toString() {
+      if (trackNumber != null) return trackNumber + " - " + name;
+      else return name; }
 }