Staff types when adding a new instrument to an existing score

When adding a new instrument to an existing score, the staff type given in the instrument template is not obeyed and a standard staff is always proposed in the Instr. dlg box.

Fixed by removing in the list of available staff types used by the Instr. dlg box a (previously needed) distinction between types coming from presets and types coming from the score. With recent changes to preset/staff types, this led to existing types not being found in the list.