MusicXML: Conversion of various types of spanners (octave, pedals, trills)

MusicXML: Conversion of various types of spanners (octave, pedals, trills)

-) Implement Octave-shift (8va, 8vb, 15ma, 15mb) and pedal spanners
-) Implement trill spanners. It seems that in MusicXML, a trill span does
   not include the trill sign, so they are duplicated in many cases.
-) Attempt to implement dashed slurs. Does not work yet, because the
   corresponding \slurDashed needs to be before the note, which the
   current code structure does not allow.
-) Implement Glissando. Explicitly setting the type to wavy does not
   work yet, because that command needs to be before the note in lilypond
   which is not yet supported by the structure of the musicxml2ly
   application.
-) Implement bends ("-\bendAfter #nr" in lilypond)
-) Use short notation where appropriate for articulations (e.g.
   "-." instead of "\staccato" or "->" instead of "\accent")

Open problem:
In MusicXML, a trill span consists of a trill sign plus a wavy-line spanner,
while in Lilypond \startTrillSpan will create both. This means that in the
future musicxml2ly needs to be changed to combine various kinds of Events.
Also, MusicXML allows a trill spanner to start and end at the same note,
which is not possible in lilypond.

Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>