When adding a new instrument to an existing score, the staff type given in the instrument template is not observed and a standard staff is always proposed in the Instr. dlg box.

Fixed by removing a (previously needed) distinction in the list of available staff types used by the Instr. dlg box between types coming from presets and types coming from the score. This led to existing types not being found in the list.
Merge pull request #473 from mgavioli/Staff_type_when_adding_a_new_instr_to_an_existing_score

Staff types when adding a new instrument to an existing score