Song: add isDownloaded() method.

src/main/java/moe/nekojimi/chords/Song.java

@@ -75,6 +75,11 @@ public class Song
         return song;
     }
 
+    public boolean isDownloaded()
+    {
+        return (location != null && location.exists() && location.canRead());
+    }
+
     public String getTitle()
     {
         return title;
@@ -191,5 +196,4 @@ public class Song
         return formats.get(0);
     }
 
-
 }