Parts of QTextCodec are duplicated

The QTextCodec class is only public when feature textcodec is enabled; its declaration is then in qtbase/src/corelib/codecs/qtextcodec.h; however, qtbase/src/corelib/codecs/qtextcodec_p.h duplicates parts of the class declaration in the #else clause for a check on the feature, presumably so that some code elsewhere can use this minimal stub of it even when the feature is disabled.

This duplicated code should probably be put into a QTextCodecBase class in a separate header, that qtextcodec.h pulls in to base QTextCodec on, while qtextcodec_p.h's #else clause pulls it in for the sake of using QTextCodec = QTextCodecBase; in support of whatever needs this minimal stub. Failing that, some other way should be found to avoid the duplication (e.g. eliminating the need for the stub in the #else clause).