Rename Tablature class to StringData

Refactoring the following names:

Classes:
Tablature => StringData

Member variables and methods:
Instrument::tablature() => stringData()
Instrument::setTablature() => setStringdata()
InstrumentTemplate::tablature => stringData
InstrumentData::_tablature => _stringData
InstrumentData::tablature() => stringData()
InstrumentData::setTablature() => setStringData()

There is no change in logic or program behaviour, only renaming.
* Fix Guitar Pro test ref. file.

* Replace "Tablature" with "StringData" in instruments.xml

* Remove support for "Tablature" tag from instrtemplate.cpp (but keep it in instrument.cpp for compatibility with existent 2.0 scores)
Merge pull request #475 from mgavioli/Rename_Tablature_class_to_StringData

Rename Tablature class to StringData