Rename Tablature class to StringData

The Tablature class refers to something which is not a tablature at all, but rather a collection of data and info about the strings of an instrument. These data are used by and for the 'real' tablature (i.e. StaffTypeTablature), but in other contexts too (see for instance the list of changed files).

This class name seems to me not optimal and potentially misleading. To improve code clarity and understandability, I propose to change it (and some related member variables and methods) as follows:

* 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()

The number of changed files is high, but there is no change in logic or program behaviour, only renaming.