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.cpp

@@ -0,0 +1,15 @@
+#include "utility/utilityQString.h"
+
+#include <QString>
+
+std::wstring utility::decodeFromUtf8(std::string s)
+{
+	QString qs = QString::fromUtf8(s.c_str());
+	return qs.toStdWString();
+}
+
+std::string utility::encodeToUtf8(std::wstring s)
+{
+	QString qs = QString::fromStdWString(s);
+	return qs.toUtf8().toStdString();
+}