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/QtCodeSnippet.h

@@ -32,6 +32,8 @@ public:
 	);
 	virtual ~QtCodeSnippet();
 
+	QtCodeFile* getFile() const;
+
 	uint getStartLineNumber() const;
 	uint getEndLineNumber() const;