Set "New episodes available" tooltip for episodes with html descriptions.

Episodes with an html description did not have a tooltip because the
description field is always empty for them. The new _text_description
field is computed at runtime and should be used for any code wanting a
text description.

Fixes #1350.

src/gpodder/gtkui/desktop/episodeselector.py

@@ -111,7 +111,7 @@ class gPodderEpisodeSelector(BuilderWidget):
             self.size_attribute = 'file_size'
 
         if not hasattr(self, 'tooltip_attribute'):
-            self.tooltip_attribute = 'description'
+            self.tooltip_attribute = '_text_description'
 
         if not hasattr(self, 'selection_buttons'):
             self.selection_buttons = {}