Avoid duplication between slur and phrasing slur engravers.

Avoid duplication between slur and phrasing slur engravers.

The original code in slur-engraver.cc contained a comment:

  This is largely similar to Phrasing_slur_engraver. Check if fixes
  apply there too.

  (on principle, engravers don't use inheritance for code sharing)

This patch violates that principle.  However, it uses neither multiple
inheritance nor virtual base classes nor templates for this, but just
a common base class with virtual functions.