Added IEnumerable properties

MidiChordDefs in OutputVoice and VoiceDef,
InputChordDefs in InputVoice.
Deleted redundant Forms in Assistant Composer

Both the PerformersOptionsForm and the
TrackInitForm have been deleted. These were
previously used to set runtime info in .mkss
files. Such options are now set in composition
algorithms.
(Also removed the 'runtime' element in  MoritzKrystalScore.xsd -- which defines the
.mkss format.)
The corresponding buttons in the
AssistantComposerMainForm have also been
removed.