logic: used wstring in FilePath constructor

* used wstring in FilePath constructor where easily possible
* added FilePath concatenate method that accepts string parameter
* implemented using only wstring in MessageStatus
* used wstring in config

src/lib_gui/utility/utilityQString.h

@@ -0,0 +1,13 @@
+#ifndef UTILITY_Q_STRING_H
+#define UTILITY_Q_STRING_H
+
+#include <string>
+
+namespace utility
+{
+	std::wstring decodeFromUtf8(std::string s);
+	std::string encodeToUtf8(std::wstring s);
+}
+
+#endif // UTILITY_Q_STRING_H
+