Selecting [undefined] TPC in Ambitus inspector causes crash.

The [undefined] item is informational only (when the ambitus range cannot be calculated, for instance when there are no notes in the staff yet) and should not be selected.

Fixed at two levels:

1) The [undefined] item is disabled and cannot be selected any longer

2) The Ambitus::layout() method is more robust to in dealing with undefined ranges.
Merge pull request #779 from mgavioli/Fix_crash_with_ambitus_undefined_tpc

Selecting [undefined] TPC in Ambitus inspector causes crash.