Add method to return a list of all optimized items

[Alindil]

Description

There can be several videos in an optimization. In the current code we only loop on one.

• Deprecates PlexServer.optimizedItem().
• Adds Optimized.items() method to return all videos that are part of the optimization.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable

[JonnyWong16]

I would rather deprecate plex.optimizedItem() and add an items() method to plexapi.media.Optimized instead. This mimics the rest of Plex API.

~plexapi.media.Optimized

    def items(self):
        """ Returns a list of all :class:`~plexapi.media.Video` objects
            in this optimized item.
        """
        key = '%s/%s/items' % (self._initpath, self.id)
        return self.fetchItems(key)