Silence FourCharCode warning

This is necessary when compiled with -Wall &
GCC_WARN_FOUR_CHARACTER_CONSTANTS
Explicitly cast to avoid conversion warnings

This is necessary when compiled with -Wall &
CLANG_WARN_SUSPICIOUS_IMPLICIT_CONVERSION
Appropriately cast inside +stringForKeycode:

This avoids callers from warnings or errors when compiled with
CLANG_WARN_SUSPICIOUS_IMPLICIT_CONVERSION