Changed ID3Tag for Track-Number to TRCK

Until now the ID3Tag for the Track-Number was read from TPOS.
The Problem is, that TPOS is the part of the set (eg disc number)
and not the track-number. TRCK is the correct tag for the track
number.

Fixes #220

api/funkwhale_api/music/metadata.py

@@ -67,7 +67,7 @@ VALIDATION = {
 
 CONF = {
     'OggVorbis': {
-        'getter': lambda f, k: f[k][0],
+        'getter': lambda f, k: f[kTr][0],
         'fields': {
             'track_number': {
                 'field': 'TRACKNUMBER',
@@ -132,7 +132,7 @@ CONF = {
         'getter': get_id3_tag,
         'fields': {
             'track_number': {
-                'field': 'TPOS',
+                'field': 'TRCK',
                 'to_application': convert_track_number
             },
             'title': {