Activated Markup.stack_priority property. NEW FUNCTIONALITY.

Set the property to a (positive, negative or zero-valued) integer.

The property is used at format time to arbitrate the order of same-directed
markup attached to a single component.

Example: foo higher than bar:

    >>> staff = Staff("c'8 d'8 e'8 f'8")
    >>> attach(Markup('foo', stack_priority=1000), staff[1])
    >>> attach(Markup('bar', stack_priority=0), staff[1])
    >>> f(staff)
    \new Staff {
            c'8
            d'8
                    - \markup {
                            \column
                                    {
                                            foo
                                            bar
                                    }
                            }
            e'8
            f'8
    }

Example 2: bar higher than foo:

    >>> staff = Staff("c'8 d'8 e'8 f'8")
    >>> attach(Markup('foo', stack_priority=0), staff[1])
    >>> attach(Markup('bar', stack_priority=1000), staff[1])
    >>> f(staff)
    \new Staff {
            c'8
            d'8
                    - \markup {
                            \column
                                    {
                                            bar
                                            foo
                                    }
                            }
            e'8
            f'8
    }

NEW: should be included in the 2.16 release notes.