Nota 783

Issue Nota-783 is solved by implementing three AuthorOperations: InsertHeadingOperation, RemoveHeadingOperation and UpdateNavigationOperation. The two first adds/romoves headings and corresponding sections in text content documents. The last updates the navigation files. The operations can be accessed throung the menu/toolbar/context menu sub menu item "H".

@cwallin or @ybk should handle the pull request, possibly assisted by @stesk