[plugins] fix adding notes using newElement() and cursor.add()

This provides a way for a plugin to add notes to existing chords.
(see implode.qml at https://github.com/heuchi/plode for a demonstration)

Added wrappers for properties pitch, tpc1 and tpc2 in Note to avoid crashes due to uninitialized pointer.
Fixed cursor.add to take care of setting a note's chord property to avoid crashes.