Open strings in StringData.

Allows to mark strings as open (non-fretted).
Defaults to fretted, keeping backward-compatibility with existing scores.
Adds an attribute `open="1"` in score (and instruments.xml) string data (defaults to "0").

Benefits:
- allows future support (also in TAB's) for string instruments with non-fretted strings (mainly but, I think, not only the lute family)
- gives correct fretting for non-fretted strings
- instrument range can be more easily and automatically linked to actual string tuning
- as this will be eventually needed for better TAB support, the score reading/writing will be already able to manage it.