PICARD-1043: Write showmovement only to MP4

Do not write the iTunes specific showmovement tag to formats other than MP4. iTunes does not support this for other formats, and there is no standard way to write this.

picard/formats/apev2.py

@@ -215,6 +215,7 @@ class APEv2File(File):
             'podcast',
             'podcasturl',
             'show',
+            'showmovement',
             'showsort',
             }
         return bool(name) and name not in unsupported_tags

picard/formats/id3.py

@@ -527,7 +527,8 @@ class ID3File(File):
 
     @classmethod
     def supports_tag(cls, name):
-        return ((name and not name.startswith("~"))
+        unsupported_tags = {'showmovement'}
+        return ((name and not name.startswith("~") and name not in unsupported_tags)
                 or name in ("~rating", "~length")
                 or name.startswith("~id3"))
 

picard/formats/vorbis.py

@@ -276,7 +276,8 @@ class VCommentFile(File):
 
     @classmethod
     def supports_tag(cls, name):
-        return bool(name)
+        unsupported_tags = {'showmovement'}
+        return bool(name) and name not in unsupported_tags
 
 
 class FLACFile(VCommentFile):