Implemented sound.length for gstreamer backend (see http://stacko…

…verflow.com/questions/2440554/how-do-i-find-the-length-of-media-with-gstreamer - media needs to be playing to find the length). See kivy#467

kivy/core/audio/audio_gstreamer.py

@@ -104,4 +104,17 @@ def _set_volume(self, volume):
             self._data.set_property('volume', volume)
         return super(SoundGstreamer, self)._set_volume(volume)
 
+    def _get_length(self):
+        if self._data is not None:
+            if self._data.get_state()[1] != gst.STATE_PLAYING:
+                self._data.set_state(gst.STATE_PLAYING)
+                try:
+                    self._data.get_state()
+                    return self._data.query_duration(gst.Format(gst.FORMAT_TIME))[0] / 1000000000.
+                finally:
+                    self._data.set_state(gst.STATE_NULL)
+            else:
+                return self._data.query_duration(gst.Format(gst.FORMAT_TIME))[0] / 1000000000.
+        return super(SoundGstreamer, self)._get_length()
+
 SoundLoader.register(SoundGstreamer)