Fontengines: De-duplicate ~6 copies of unsigned int getChar()

The function static inline unsigned int getChar(const QChar *str, int &i, const int len) has been copy pasted into each fontengine:

• qfontengine_ft.cpp
• qfontengine_qpf.cpp
• qfontengine_qpa.cpp (since Qt 4.8)
• qfontengine_qws.cpp
• qfontengine_s60.cpp (this is not 'static')
• qfontengine_win.cpp

Maybe, there is a reason for having them duplicated. But putting them into a central place, such as qfontengine.h, may be worth a thought.