[ftinspect] Fix default named instance handling.

Now the named instance selector no longer doubles the default named instance.
`FT_Get_Default_Named_Instance` is used to get the default NI index.
The default one is labeled as " (default)".

* src/ftinspect/widgets/tripletselector.cpp: Properly handle the new 1-based
  named instance indices. Label the default NI correctly.

* src/ftinspect/engine/engine.cpp, src/ftinspect/engine/engine.hpp:
  Add `Engine::defualtNamedInstanceIndex`.

* src/ftinspect/engine/mmgx.cpp: Change comments.