Fix issues 75 and 1256: Allow multiple concurrent slurs

Fix issues 75 and 1256: Allow multiple concurrent slurs

Rewrite the Slur_engraver and the Phrasing_slur_engraver to support
multiple concurrent slurs. The default lilypond syntax using parentheses
still supports only one slur at a time, but by adding a spanner-id property
to the (Phrasing)SlurEvent music expression, one can create multiple
concurrent slurs, each with a different spanner-id.

This finally allows appoggiaturas and acciaccaturas (which both create a
slur from the grace note the the next note) to be placed inside a slur.

If we observe a new slur start while a slur is already present, we now
totally ignore the new slur event, so it does not influence the appearance
of the existing slur (bug 1256)