Renamed private Spanner._leaves property.

OLD: Spanner._leaves
NEW: Spanner._get_leaves()

Explanation: the code involves (potentially extensive) iteration. The old
implementation as a property conceals the fact that the call could be costly.
The new implementation as a method avoids this.
Taught TextSpanner about LineSegment and Markup objects.
Added new Arrow class to indicatortools. NEW.

Example:

    >>> staff = Staff("c'4 d'4 e'4 f'4")
    >>> start_markup = Markup('pont.').upright()
    >>> stop_markup = Markup('ord.').upright()
    >>> arrow = indicatortools.Arrow()

    >>> attach(start_markup, staff[0], is_annotation=True)
    >>> attach(stop_markup, staff[-1], is_annotation=True)
    >>> attach(arrow, staff[0])
    >>> attach(spannertools.TextSpanner(), staff[:])

    >>> f(staff)
    \new Staff {
            \once \override TextSpanner.arrow-width = 0.25
            \once \override TextSpanner.bound-details.left.stencil-align-dir-y = 0
            \once \override TextSpanner.bound-details.left.text = \markup {
                    \concat
                            {
                                    \upright
                                            pont.
                                    \hspace
                                            #0.25
                            }
                    }
            \once \override TextSpanner.bound-details.right.arrow = ##t
            \once \override TextSpanner.bound-details.right.padding = 1.5
            \once \override TextSpanner.bound-details.right.stencil-align-dir-y = 0
            \once \override TextSpanner.dash-fraction = 1
            c'4 \startTextSpan
            d'4
            e'4
            f'4 \stopTextSpan ^ \markup {
                    \upright
                            ord.
                    }
    }

See the arrow API entry for notation.

NEW: should be included in the 2.16 release notes.
Wrote Arrow API examples. NEW.

NEW: Arrow should now be included in the 2.16 release notes.
Taught TextSpanner more about Markup and LineSegment objects.
Merged master.