Improved Spanner._apply_overrides().

You can now magically persist complex text spanners like this:

    white_flautando_spanner = spannertools.TextSpanner(
            overrides = {
                    'text_spanner__dash_period': '1.5',
                    'text_spanner__bound_details__right_broken__padding': '0',
                    'text_spanner__bound_details__left_broken__text': "markuptools.Markup((markuptools.MarkupCommand('italic','(fl.)'),markuptools.MarkupCommand('hspace',1)))",
                    'text_spanner__bound_details__left__stencil_align_dir_y': '0',
                    'text_spanner__bound_details__right__text': "markuptools.Markup((markuptools.MarkupCommand('draw-line',schemetools.SchemePair(0, -1)),))",
                    'text_spanner__bound_details__right_broken__text': 'None',
                    'text_spanner__bound_details__left__text': "markuptools.Markup((markuptools.MarkupCommand('italic','\\xe2\\x80\\x9cwhite\\xe2\\x80\\x9d flautando'),markuptools.MarkupCommand('hspace',1)))",
                    'text_spanner__bound_details__right__padding': '1',
                    'text_spanner__bound_details__left__padding': '-1',
                    'text_spanner__dash_fraction': '0.25',
            }
    )

Test added.