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/qt/view/QtUndoRedoView.cpp

@@ -68,5 +68,6 @@ void QtUndoRedoView::updateHistory(const std::vector<SearchMatch>& searchMatches
 void QtUndoRedoView::setStyleSheet()
 {
 	m_widget->setStyleSheet(utility::getStyleSheet(
-		ResourcePaths::getGuiPath().concatenate(FilePath("undoredo_view/undoredo_view.css"))).c_str());
+		ResourcePaths::getGuiPath().concatenate(L"undoredo_view/undoredo_view.css")
+	).c_str());
 }