Implement presentation mode

• The book preview should be opened with F5 key or with View-> Presentation mode
• The book should be opened in preview mode in full screen
• Only the page should be shown (and not the frames that go outside of it), zoomed to fit in the display. Additionaly, equal black areas should appear on the left and right or top and bottom. This depends on aspect ratios of the display and book.
• Page down and page up buttons should go to previous/next pages
• Space button should go to next page
• left/right kbd buttons should go to previous/next page