logic: Return to last scroll position in code view on undo

* fixed scrolling on undo
* refactored autoscrolling in code view
* save scroll position of user with MessageScrollCode

src/lib_gui/qt/element/QtCodeArea.h

@@ -71,6 +71,7 @@ public:
 	uint getEndLineNumber() const;
 
 	std::shared_ptr<TokenLocationFile> getTokenLocationFile() const;
+	QtCodeFile* getFile() const;
 
 	void lineNumberAreaPaintEvent(QPaintEvent* event);
 	int lineNumberDigits() const;