#145 Setting the page number offset

[szhorvat]

For most books, the page numbers appearing *in* the book don't match the DJVU page numbers. E.g. page 60 of the book may be the 73rd page of the DJVU document. WinDjView can already store information about individual documents (e.g. bookmarks, last view of the document, etc.) It would be tremendously useful if it were possible to set a page number offset (so, with the above example, it is possible to go to page 60 directly by typing "60" instead of 73), and this page number offset were remembered by the viewer for each document.