Fix overlapping EHeaderFlags values (#902)

Instrumental (5) and Vocals (6) overlapped with Title|MP3 and
Artist|MP3 in the [Flags] enum. A song file with #INSTRUMENTAL or
#VOCALS therefore passed the mandatory-tag checks even when #TITLE,
#ARTIST or #MP3 were missing, leaving Audio empty.

Give each flag its own bit (Instrumental = 64, Vocals = 128).

Co-authored-by: Florian Ostertag <flokuep@users.noreply.github.com>

VocaluxeLib/Songs/CSong.cs

@@ -44,11 +44,11 @@ namespace VocaluxeLib.Songs
         Title = 1,
         Artist = 2,
         MP3 = 4,
-        Instrumental = 5,
-        Vocals = 6,
         Bpm = 8,
         MedleyStartBeat = 16,
-        MedleyEndBeat = 32
+        MedleyEndBeat = 32,
+        Instrumental = 64,
+        Vocals = 128
     }
 
     public enum EDataSource