Double page view

[xgi]

Add an option for displaying 2 pages in the reader side-by-side.

When the option is enabled, there would likely be 2 visible ImageView's. The ReaderController keeps track of the "current page number" -- I would be fine with simply using the first of the 2 visible pages as that number, and the subsequent page is displayed if available.

This change would also affect how the "Fit Height/Width/etc" buttons work.

(based off #12)