Made (<) and (>) methods public.

OLD:

    MaterialPackageWrangler._navigate_to_previous_asset()
    MaterialPackageWrangler._navigate_to_next_asset()
    SegmentPackageWrangler._navigate_to_previous_asset()
    SegmentPackageWrangler._navigate_to_next_asset()

NEW:

    MaterialPackageWrangler.go_to_previous_asset()
    MaterialPackageWrangler.go_to_next_asset()
    SegmentPackageWrangler.go_to_previous_asset()
    SegmentPackageWrangler.go_to_next_asset()

UI: the goal is to have one public method for each user command.