Added `tail` property to LogicalTie.

This mirrors LogicalTie's `head` property.
Streamlined TrillSpanner.
Improved trill formatting in LilyPondFormatManager.

TrillSpanner contributions to the `right` format slot are now always placed at
the end of that slot's contributions.

This is because the "pitch" which follows a `\startTrillSpan` command in a
pitched trill is really a note-event as far as LilyPond is concerned. If a
tie-event follows that note-event, the note-event will consume the tie, and no
tie grob will be printed.

By always guaranteeing that trills format farthest to the right, we prevent
ties from disappearing.
Added new spanner tool: ComplexTrillSpanner.

- creates a spanned trill on every pitched logical tie it covers
- ends all trills cleanly via a `<> \stopTrillSpan` event
- allows for relative trill pitches by using an interval rather than
    absolute pitch for its trill pitch
- avoids rests