Deprecate C code generator 'IdentifierSettings' feature and introduce 'Naming' feature instead.

[nyssen]

While the Java generator offers a 'Naming' feature, the C code generator offers an 'IdentiferSettings' feature, which has identical scope. We should rename the C generator feature to use 'Naming' as well, so code generator languages are more consistent.

[andreasmuelder]

Instead of renaming, we should deprecate the IdentifierSettings feature and add an additional naming feature for the upcoming version

[nyssen]

That would be even better, as it allows an easy migration path.