Change some RString methods to free functions

These ones aren't a std::string method. Doing this helps the RString to
std::string migration.

src/ScreenOptionsExportPackage.cpp

@@ -190,7 +190,7 @@ static RString ReplaceInvalidFileNameChars( RString sOldFileName )
 		'<', '>', ',', '?', '/'
 	};
 	for( unsigned i=0; i<sizeof(charsToReplace); i++ )
-		sNewFileName.Replace( charsToReplace[i], '_' );
+		Replace(sNewFileName, charsToReplace[i], '_');
 	return sNewFileName;
 }