Implement page wise scrolling for all zoom levels

This PR implements a page wise scrolling mode in pdf.js.
Currently switching between scrolling modes is done using the context menu.

__Things left to do in this PR:__
- [x] Wait for PR #2792 to land.
- [x] Replace the context menu items with toolbar buttons, placed in the secondary toolbar.
- [x] Provide icons for the buttons. (Thanks to @timvandermeij for providing these!)
- [ ] Add localization strings (at least for English).
- [ ] Wait for PR #3723 to land.
- [x] __Fix searching in page wise scroll mode.__
- [x] Move as much as possible of the code in this PR to its own file, to avoid increasing the size of `viewer.js` to much.

(This PR replaces #2682.)