Issue 2584 (redo 1967): please make partcombine merge slurs

Issue 2584 (redo 1967): please make partcombine merge slurs

The partcombiner does not really bother about keeping the number of
generated start and end slur events matched, so this attempts to cope
by implementing the following behavior:

a) multiple slur starts on the same moment are not an error but the
   same as one.
b) multiple slur ends on the same moment are not an error but the same
   as one.
a2) there will be a slur with direction UP if there is at least one
   such start event, and there will be a slur with direction DOWN if
   there is at least one such end event.  This might imply a double
   slur, but for ending it, a single slur end is sufficient.

Consequently, a^(_( c)) (the second closing paren not required, just
added for prettiness) will add a double slur.