Add WiiSaveBanner class

This class is similar to the BannerLoaderWii class that was
removed in ee694e32.

Source/Core/Common/StringUtil.h

@@ -125,6 +125,7 @@ std::string CP1252ToUTF8(const std::string& str);
 std::string SHIFTJISToUTF8(const std::string& str);
 std::string UTF8ToSHIFTJIS(const std::string& str);
 std::string UTF16ToUTF8(const std::wstring& str);
+std::string UTF16BEToUTF8(const char16_t* str, size_t max_size);  // Stops at \0
 
 #ifdef _WIN32