[WIP] UI for viewing and setting default preferences

This patch is a follow-up of PR #3850, and it contains a mock-up of a UI for viewing and setting the default preferences from within the viewer. The preferences are accessed through a button placed in the secondary toolbar.

__Please note: This PR is *only* submitted to gauge the interest in this feature, it is by no means ready to be merged in its current form!__

*If* this functionality should be added to PDF.js, the following is an incomplete list of things that would need to be fixed before this could be merged:
- [ ] Improve the visual appearance of the UI. (Currently it looks horrible.)
- [ ] Provide more descriptive information about what the different preferences actually does.
- [ ] Localization of the strings (at least for English).
- [ ] General improvements of the code quality. (This is just a quick hack.)

I didn't really want to spend a lot of time on this, before knowing whether this functionality is wanted or not, hence the ugly appearance. Despite this, the basic functionality should be OK, so it should be possible to test this.

/cc @brendandahl