Fixed names utf8ToWide and wideToUtf8; added convenience functions

taking std:;string and core::stringw as parameter (removed .c_str()
in many function calls).

src/utils/string_utils.cpp

@@ -686,7 +686,7 @@ namespace StringUtils
 
     // ------------------------------------------------------------------------
 
-    std::string wide_to_utf8(const wchar_t* input)
+    std::string wideToUtf8(const wchar_t* input)
     {
         static std::vector<char> utf8line;
         utf8line.clear();
@@ -695,11 +695,11 @@ namespace StringUtils
         utf8line.push_back(0);
 
         return std::string(&utf8line[0]);
-    }
+    }   // wideToUtf8
 
     // ------------------------------------------------------------------------
 
-    irr::core::stringw utf8_to_wide(const char* input)
+    irr::core::stringw utf8ToWide(const char* input)
     {
         static std::vector<wchar_t> utf16line;
         utf16line.clear();
@@ -708,7 +708,7 @@ namespace StringUtils
         utf16line.push_back(0);
 
         return irr::core::stringw(&utf16line[0]);
-    }
+    }   // utf8ToWide
 
     // ------------------------------------------------------------------------
     /** Converts a version string (in the form of 'X.Y.Za-rcU' into an