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/element/QtCodeFile.h

@@ -30,7 +30,6 @@ public:
 	void setModificationTime(const TimeStamp modificationTime);
 
 	const FilePath& getFilePath() const;
-	std::string getFileName() const;
 
 	const QtCodeFileTitleBar* getTitleBar() const;