fix non-ansi characters issue in Text

sfText_[sg]etUnicodeString functions deal with raw codepoints on four
bytes unsigned numbers, their “encoding” is sort of UTF-32 whose
endianness is implicitely that of the local architecture. This isn't
what CString is meant to do, therefore, Ptr Word32 is the correct FFI
type. Also, Char maps to Word32 without problems.
add a demo that displays non-ansi characters