Support per-page scrolling

[waddlesplash]

When set to "Page Fit" and any scroll action is triggered (down-scrollbar arrow, mid-click, scrollwheel, etc.), PDF.js should scroll a whole page and not just a few pixels. This is the behavior Adobe Reader has, and it feels appropriate.

[Snuffleupagus]

I've got a working prototype for this, there is just a few small issues left to deal with. After fixing these and improving my code, I should be able to have a PR ready some time later this week.

[der-stefan]

Oh yes, full page zomming and scrolling is a long-awaited feature. If you look at presentation slides, every mouse wheel tick should scroll one page. The new fullscreen mode does this correctly, but I would also like to have this feature in non-fullscreen mode.

BTW: That is the main cause why I open the PDFs in Acrobat and not natively in Firefox with pdf.js.