Slurs crossing a line break

[Heikki Johannes Junes]

The length of the slurs crossing a line break seems to be rather good. However,
it seems like the notes are now tied to an invisible note that is not present,
see regression tests:

  accidental-tie.ly
  completion-heads.ly
  lyric-extender-broken.ly
  slur-stem-broken.ly

Would it be more beatiful, if the length of the slur were (approximately)
doubled, and the slur were at the middle point (shown as `o´ in the examples 
below):

\__o__/

----.
     \
      o
       \
        .
        |
        |

The trend of the slur cut at the middle point would be same both in the end and
beginning of the two subsequent lines.

By implementing this, one could then use the same mechanism to produce slurs
having both up and down directions "a^( a'_)". (See especially the `broken´
example in Tips&Tricks: slur-cross-staff.ly.)

A slur break in three lines does not look nice either (see regression test:
slur-broken-trend.ly), but for that case I do not have an obvious suggestion.

-- 
  Heikki Junes