Leaf can now format grob revert format contributions.

However, leaves never receive any grob revert format contributions from the
override() mechanism, so nothing changes for normal usage.
Implemented `_lilypond_format_bundle` on LilyPondGrobOverride.

You can now attach LilyPond grob override objects to leaves and they will
format properly.

    >>> override = lilypondnametools.LilyPondGrobOverride()
    >>> staff = Staff("c'4 d'4 e'4 f'4")
    >>> attach(override, staff[1])
    >>> f(staff)
    \new Staff {
            c'4
            \override NoteHead.color = red
            d'4
            \revert NoteHead.color
            e'4
            f'4
    }

Some work still needs to be done for grob override attachment on containers.
LilyPondGrobOverrides may now be attached to containers.

    >>> override = lilypondnametools.LilyPondGrobOverride()
    >>> staff = Staff("c'4 d'4 e'4 f'4")
    >>> attach(override, staff)
    >>> f(staff)
    \new Staff \with {
            \override NoteHead.color = red
    } {
            c'4
            d'4
            e'4
            f'4
    }