Allow Ctrl+R when "No new episodes" is displayed

When the feed update has finished, and the Desktop
version of gPodder shows "No new episodes" in the
progress bar (and the "OK" button on it), the
keyboard shortcut Ctrl+R did not work, because the
menu item was disabled.

src/gpodder/gui.py

@@ -2833,6 +2833,7 @@ class gPodder(BuilderWidget, dbus.service.Object):
             self.feed_cache_update_cancelled = True
             self.btnCancelFeedUpdate.show()
             self.btnCancelFeedUpdate.set_sensitive(True)
+            self.itemUpdate.set_sensitive(True)
             if gpodder.ui.maemo:
                 # btnCancelFeedUpdate is a ToolButton on Maemo
                 self.btnCancelFeedUpdate.set_stock_id(gtk.STOCK_APPLY)