First step in reducing unneeded full refreshes of treeChannels
libpodcasts.py: Moved a bunch of code from channels_to_model to
update_channel_model_by_iter, a new method which allows updating of
individual iters in a model.
gui.py: Added only_selected_channel option to updateComboBox which makes it
only update the selected channel's iter. This saves a bit of time when doing
simple tasks like marking an episode as played. So far only playback_episode
and for_each_selected_episode_url take advantage of this option.