issue 5413: X-aligning problem with chords containing unisons

issue 5413: X-aligning problem with chords containing unisons

Changes to be committed:

modified:   ../lily/stem.cc   Stem::extremal_heads
- To avoid confusion, process noteheads in <>  from left to right
- lowest note: return FIRST unison note (punctum saliens!)
- highest note: return LAST unison note
Separate DOWN/UP processing now (because of < vs. >=).
Result: consistent spacing plus correct first/last/main notehead detection.

new file:   ../input/regression/chord-X-align-on-main-noteheads.ly
- Regression test added

[Be-3, 2018-09-05 20:05:12]

Please review.

Thanks,
Torsten

[Carl, 2018-09-10 17:39:24]

LGTM.

Carl