[trunk] instrument string tunings are forced to be in sequential order

Context: trunk rev. 5390

A) In the "Edit string data" dlg box, it is not possible to arrange string tunings in non-sequential order.
B) When tuning data for an instrument are read from instruments.xml, they are also sorted in sequential order.

String instruments may have non-sequential string tunings, like banjo or theorbo.

M.