PICARD-259: Execute tagger script when file gets moved to track

This allows using file specific metadata and variables (such as e.g. %_bitrate%) to be used in tagger script.

Also fixes PICARD-23

picard/track.py

@@ -170,7 +170,12 @@ class Track(DataObject, Item):
     def update_file_metadata(self, file):
         if file not in self.linked_files:
             return
-        file.copy_metadata(self.metadata)
+        # Run the scripts for the file to allow usage of
+        # file specific metadata and variables
+        metadata = Metadata(file.metadata)
+        metadata.update(self.orig_metadata)
+        self.run_scripts(metadata)
+        file.copy_metadata(metadata)
         file.metadata['~extension'] = file.orig_metadata['~extension']
         file.update(signal=False)
         self.update()
@@ -189,6 +194,16 @@ class Track(DataObject, Item):
         if self.item.isSelected():
             self.tagger.window.refresh_metadatabox()
 
+    @staticmethod
+    def run_scripts(metadata):
+        for s_name, s_text in enabled_tagger_scripts_texts():
+            parser = ScriptParser()
+            try:
+                parser.eval(s_text, metadata)
+            except ScriptError:
+                log.exception("Failed to run tagger script %s on track", s_name)
+            metadata.strip_whitespace()
+
     def update(self):
         if self.item:
             self.item.update()
@@ -403,14 +418,7 @@ class NonAlbumTrack(Track):
         self.orig_metadata.copy(m)
         self._customize_metadata()
         run_track_metadata_processors(self.album, m, recording)
-        for s_name, s_text in enabled_tagger_scripts_texts():
-            parser = ScriptParser()
-            try:
-                parser.eval(s_text, m)
-            except ScriptError:
-                log.exception("Failed to run tagger script %s on track", s_name)
-            m.strip_whitespace()
-
+        self.run_scripts(m)
         self.loaded = True
         self.status = None
         if self.callback: