Extended Session.display_variables().

Ouput now displays controller stack with one controller per line.

Example:

    > sdv

    command_history: ['ssx', 'red', 'm', '1', 'sdv']
    controller_stack:
        ScoreManager()
        ScorePackageManager('/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score')
        MaterialPackageWrangler()
        MaterialManager('/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/materials/magic_numbers')
        <Menu 'material manager' (19)>
    current_score_package_manager: ScorePackageManager('/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score')
    hide_hidden_commands: True
    hide_next_redraw: False
    is_autoadding: False
    is_in_editor: False
    last_asset_path: '/Users/trevorbaca/Documents/abjad/scoremanager/scores/red_example_score/materials/magic_numbers'
    scores_to_display: 'example'

    Press return to continue.