Model: Day, month and year for custom filename (bug 1768)

Users can now use the following fields to customize the
sync filename when using MP3 player device sync:

  {episode.pubdate_year} - Year of publication (without century)
  {episode.pubdate_month} - Month of publication (01-12)
  {episode.pubdate_day} - Day of publication (01-31)

In addition to these fields, {episode.sortdate} is still
provided and gives YYYY-MM-DD as result (just like before).

src/gpodder/model.py

@@ -682,9 +682,24 @@ def cute_pubdate(self):
 
     pubdate_prop = property(fget=cute_pubdate)
 
+    def published_datetime(self):
+        return datetime.datetime.fromtimestamp(self.published)
+
     @property
     def sortdate(self):
-        return str(datetime.datetime.fromtimestamp(self.published).strftime('%F'))
+        return self.published_datetime().strftime('%F')
+
+    @property
+    def pubdate_day(self):
+        return self.published_datetime().strftime('%d')
+
+    @property
+    def pubdate_month(self):
+        return self.published_datetime().strftime('%m')
+
+    @property
+    def pubdate_year(self):
+        return self.published_datetime().strftime('%y')
 
     def is_finished(self):
         """Return True if this episode is considered "finished playing"