Renamed "data-sb-mark" to "data-sb-obj". Added data-sb-obj to ScrapBook generated css tags.
Fixed a problem that some Scrapbook additions (such as sticky comments) cannot be correctly removed, and a problem that some objects are not correctly removed with "Remove All Scrapbook Additions" command.

- merge "sticky-absolute" and "sticky-relative" to "sticky", no longer use "sticky-header" and "sticky-footer"
Undo button can now be disabled whenever no history is unavailable. Editing inline annotation and editing or moving sticky annotation can now be undone.