Fix out-of-range page and tweak swipe

* Avoid PageUpdate from receiving out-of-range page

Otherwise, `current_page` can acquire negative values, or values greater than the page count, when the user tries to go back the first page or past the last page.

*  Restore north and south swipe gestures

Might be useful in pages covered with links, which make it difficult to tap in the correct point for flipping the page.