Two-Up View Redux

Building on what @Moistly started in #2395 (to fix #1475). Note that the following things might still be an issue:

## To-Do (before merging)
 - [x] Have "bookmark" option in toolbar capture the two-page settings
 - [x] Store in settings that the page was viewed in 2up mode (and restore the mode the next time the PDF is viewed)
 - [x] Remove shadow between two pages
 - [x] Three-page view happens occasionally
 - [x] Full-screen mode broken with two-up active
 - ~~When two-up is active and the zoom is set to 'page-fit', the vertical alignment of the pages becomes wrong. In this configuration, the uppermost pixels of the next pages become visible in the bottom of the screen. Note that I can only reproduce this if "Show Cover Page in Two Page View" is toggled on.~~
 - [x] In two-up view the margin to the left of the pages is always bigger than the right one, i.e. the pages gets placed of-centre horizontally in the viewerContainer. Edit: From looking at the HTML/CSS with the inspector in Firebug, it's clear that the margins are set totally wrong. Perhaps it would be good to instead define a special CSS class that gets invoked when switching to two-up. (This might also fix full screen mode.)
 - [x] There seems to be some issue regarding the rendering of pages when toggling "Show Cover Page in Two Page View". Until you actually scroll, or change page, the pages won't render.
    The above also sometimes happen when switching to/from two-up, but less frequently.
 - [x] It seems that the right hand page in two-up mode doesn't render as it should when scrolling. Observing how PDFView.getVisiblePages().views changes while scrolling down in a document probably explains a lot. Currently PDFView.getVisiblePages().views doesn't always correspond to what you actually see on the screen.
 - [x] When zooming in, the horizontal scrollbar don't get big enough, so some of the pages gets clipped of particular on the left side. This might be related to the margins, see above.
 - ~~When twoup is turned on, the cover page may randomly only draw half the page **and never draw the rest unless the page is reloaded**~~ *I can no longer reproduce.*
 - [x] The next-page button does not always act as expected.