Also removes the default guitar string data, which does not make sense for non-stringed instruments (stringed instruments are assumed to have proper string data in instruments.xml anyway)
Renames also the files hoding the class itself:
- libmscore/tablature.cpp => stringdata.cpp
- libmscore/tablature.h => stringdata.h
No actual change in the code, only updated references to stringdata.h in #include's.