add EncodeUTF8String, DecodeUTF8String

[salix5]

Change the function definitions to:

static int EncodeUTF8String(const wchar_t* wsrc, char* str, int size);
template<size_t N>
static int EncodeUTF8(const wchar_t* src, char(&dst)[N]);

static int DecodeUTF8String(const char* src, wchar_t* wstr, int size);
template<size_t N>
static int DecodeUTF8(const char* src, wchar_t(&dst)[N]);

Now the arguments can be pointer as before. The files in gframe still use EncodeUTF8, DecodeUTF8.

@mercury233 @purerosefallen