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.cpp

@@ -110,6 +110,11 @@ QtCodeSnippet::~QtCodeSnippet()
 {
 }
 
+QtCodeFile* QtCodeSnippet::getFile() const
+{
+	return m_codeArea->getFile();
+}
+
 uint QtCodeSnippet::getStartLineNumber() const
 {
 	return m_codeArea->getStartLineNumber();