https://github.com/coolwanglu/pdf2htmlEX/issues/180

Better name for the key handler.
https://github.com/coolwanglu/pdf2htmlEX/issues/180

Call e.preventDefault() when the key was handled.
https://github.com/coolwanglu/pdf2htmlEX/issues/180

Scroll pages with alt-page up/down, not page up/down.

Implement screen scrolling for page up/down.

Fix comment about which key is up (code was right, but comments were inverted :D)