fixed typo
- the old fix would produce subsection slides with empty classList, instead properly filling them with 'past' or 'future'; this commit introduces a better fix for this: Solves an edge case where the previous slide maintains the 'present' class when navigating between adjacent vertical stacks

- when keyboard navigating the slides while having contenteditable areas, now catch SHIFT-TAB and TAB moving focus to elements in HIDDEN slides and forcing focus to move to the document itself instead so that the keys are processed by the slides navigation logic like they should.